feat(shell): global back and forward in the top bar

The history stack has been global since the Android back gesture landed
-- every navigation is an entry and `popstate` restores any of them in
either direction. What the report describes as "back is tab-scoped" is
that the only way back was a detail view's own button, which leaves the
screen with the view it belongs to: click over to Tracks and the album
you were reading is still one entry away with nothing on screen saying
so.

`<nav-history>` is that affordance, plus `nav.back` / `nav.forward` on
Alt+Left / Alt+Right -- the browser's own combination, and clear of the
bare arrows that seek, since a binding matches on its full canonical
string.

Forward is not back negated, which is why the old `pushedEntries`
counter is gone rather than extended: `popstate` carries no direction
and fires identically both ways, so one counter decremented on every
pop reads a forward as a second back. Each entry carries its index and
the shell keeps the current one and a high-water mark, which also
survives a jump of more than one.

The buttons dispatch the events the rest of the app already dispatches
rather than calling `history` themselves -- the shell owns the guard
that stops a press at the root leaving the app, and a second caller
reaching for history is how the old `navStack` came to disagree with
the platform.

Below 900px the control stands down: the top bar is what runs out of
room first below that, and nothing becomes unreachable -- the shortcuts
are global at every width and the phone has the platform's gesture.

Closes #6

frontend/src/assets/icons/fa/solid/arrow-right.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 7.3.1 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2026 Fonticons, Inc. --><path fill="currentColor" d="M502.6 278.6c12.5-12.5 12.5-32.8 0-45.3l-160-160c-12.5-12.5-32.8-12.5-45.3 0s-12.5 32.8 0 45.3L402.7 224 32 224c-17.7 0-32 14.3-32 32s14.3 32 32 32l370.7 0-105.4 105.4c-12.5 12.5-12.5 32.8 0 45.3s32.8 12.5 45.3 0l160-160z"/></svg>

frontend/src/components/nav-history/nav-history.ts

@@ -0,0 +1,133 @@
+import { LitElement, html, css } from 'lit';
+import { customElement } from 'lit/decorators.js';
+import '@awesome.me/webawesome/dist/components/icon/icon.js';
+import { designTokens } from '../../styles/tokens.css';
+import { HistoryController } from '@store/controllers/history-controller';
+
+/**
+ * Global back and forward, in the top bar (#6).
+ *
+ * **The stack was already global; the affordance was not.** Every
+ * navigation has been a history entry since the Android back gesture
+ * landed, and `popstate` restores any of them in either direction --
+ * `back-navigation.spec.ts` has asserted `goForward()` since it was
+ * written. What the report describes as "back is tab-scoped" is that
+ * the *only* way back was a detail view's own button, which vanishes
+ * the moment you leave for another tab: the album you were reading is
+ * still one entry away, and nothing on screen says so or offers it.
+ *
+ * Four things about this are load-bearing.
+ *
+ * **It asks the shell rather than the History API.** `history.length`
+ * counts entries this app did not push and never shrinks, and there is
+ * no way to ask where in the list you are -- so a control derived from
+ * it is confidently wrong at both ends. `historyStore` is the shell's
+ * own numbering.
+ *
+ * **A control that cannot act is `disabled`, not hidden.** This is the
+ * one place in the app where that is right rather than the fault
+ * `library-status-indicator` was: back and forward are a *pair* whose
+ * positions the user learns, and a button that disappears at the end
+ * of the list moves the other one under the cursor. It is also what
+ * every browser does, which is the whole design brief here.
+ *
+ * **The buttons dispatch the events the rest of the app already
+ * dispatches**, `navigate-back` and `navigate-forward`, rather than
+ * calling `history.back()` themselves. The shell owns the guard -- one
+ * press is one entry, and at the root there is nothing of ours to go
+ * back to -- and a second caller reaching for `history` directly is
+ * how the old `navStack` came to disagree with the platform.
+ *
+ * **It is desktop chrome.** Below 600px the phone has a system back
+ * gesture (and, on Android, a hardware/gesture Back that this app
+ * hooks), the top bar is 3.25em with three other things in it, and two
+ * more 32px targets there would be the first thing to overflow. Hidden
+ * by `index.css` at that width, next to the rest of the phone header's
+ * concessions.
+ */
+@customElement('nav-history')
+export class NavHistory extends LitElement {
+    private historyCtrl = new HistoryController(this);
+
+    static override styles = [designTokens, css`
+        :host {
+            display: flex;
+            align-items: center;
+            gap: 0.25em;
+            /* A grid item's implicit minimum is its content; this one
+               genuinely cannot shrink, so it says so rather than
+               letting the header widen the body. */
+            flex: 0 0 auto;
+        }
+
+        button {
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            width: 2em;
+            height: 2em;
+            padding: 0;
+            border: none;
+            border-radius: 50%;
+            background: transparent;
+            color: var(--yj-text-primary, #f8f9fa);
+            cursor: pointer;
+            font-size: 1em;
+        }
+
+        button:hover:not(:disabled) {
+            background-color: var(--yj-bg-overlay, #495057);
+        }
+
+        button:focus-visible {
+            outline: 2px solid var(--yj-accent, #ffd43b);
+            outline-offset: 2px;
+        }
+
+        button:disabled {
+            /* Not a contrast failure: a disabled control is exempt from
+               1.4.3, and the pair has to read as unavailable rather
+               than merely quiet. */
+            color: var(--yj-text-tertiary, #868e96);
+            cursor: default;
+        }
+    `];
+
+    private go(direction: 'back' | 'forward') {
+        this.dispatchEvent(new CustomEvent(`navigate-${direction}`, {
+            bubbles: true,
+            composed: true,
+        }));
+    }
+
+    override render() {
+        const { canBack, canForward } = this.historyCtrl.depth;
+
+        return html`
+            <button
+                type="button"
+                data-testid="history-back"
+                aria-label="Back"
+                ?disabled=${!canBack}
+                @click=${() => this.go('back')}
+            >
+                <wa-icon name="arrow-left"></wa-icon>
+            </button>
+            <button
+                type="button"
+                data-testid="history-forward"
+                aria-label="Forward"
+                ?disabled=${!canForward}
+                @click=${() => this.go('forward')}
+            >
+                <wa-icon name="arrow-right"></wa-icon>
+            </button>
+        `;
+    }
+}
+
+declare global {
+    interface HTMLElementTagNameMap {
+        'nav-history': NavHistory;
+    }
+}

frontend/src/icons/names.txt

@@ -19,6 +19,7 @@ regular/heart
 regular/star
 solid/arrow-down-wide-short
 solid/arrow-left
+solid/arrow-right
 solid/arrow-rotate-right
 solid/arrows-rotate
 solid/arrow-up-short-wide

frontend/src/services/keyboard-shortcut-service.ts

@@ -400,6 +400,20 @@ async function dispatch(action: string): Promise<void> {
             break;
         }
 
+        // The keyboard half of #6. It dispatches the same events the
+        // header's buttons and the detail views' own back buttons do,
+        // rather than calling `history.back()` here: the shell owns the
+        // guard that stops a press at the root leaving the app, and a
+        // second caller reaching for `history` directly is how the old
+        // `navStack` came to disagree with the platform.
+        case 'nav.back':
+            document.dispatchEvent(new CustomEvent('navigate-back'));
+            break;
+
+        case 'nav.forward':
+            document.dispatchEvent(new CustomEvent('navigate-forward'));
+            break;
+
         case 'nav.queue': {
             const queuePanel = document.getElementById(
                 'queue-panel',

frontend/src/services/shortcut-meta.ts

@@ -103,6 +103,18 @@ export const SHORTCUT_META: Record<string, ShortcutMeta> = {
             scope: 'global',
             defaultKey: 'Q',
         },
+        'nav.back': {
+            label: 'Back',
+            category: 'Navigation',
+            scope: 'global',
+            defaultKey: 'Alt+Left',
+        },
+        'nav.forward': {
+            label: 'Forward',
+            category: 'Navigation',
+            scope: 'global',
+            defaultKey: 'Alt+Right',
+        },
         'app.shortcuts': {
             label: 'Keyboard Shortcuts',
             category: 'App',

frontend/src/store/controllers/history-controller.ts

@@ -0,0 +1,40 @@
+import type {
+    ReactiveController,
+    ReactiveControllerHost,
+} from 'lit';
+import { historyStore, type HistoryDepth } from '../history-store';
+
+/**
+ * HistoryController connects a Lit component to the HistoryStore.
+ *
+ * Usage in a component:
+ *
+ *   private historyCtrl = new HistoryController(this);
+ *
+ *   render() {
+ *     const { canBack } = this.historyCtrl.depth;
+ *   }
+ */
+export class HistoryController implements ReactiveController {
+    private host: ReactiveControllerHost;
+    private unsubscribe?: () => void;
+
+    constructor(host: ReactiveControllerHost) {
+        this.host = host;
+        host.addController(this);
+    }
+
+    hostConnected(): void {
+        this.unsubscribe = historyStore.subscribe(() => {
+            this.host.requestUpdate();
+        });
+    }
+
+    hostDisconnected(): void {
+        this.unsubscribe?.();
+    }
+
+    get depth(): HistoryDepth {
+        return historyStore.get();
+    }
+}

frontend/src/store/history-store.ts

@@ -0,0 +1,66 @@
+/**
+ * How far the session can go back and forward.
+ *
+ * The History API exposes `length` and nothing useful: it counts
+ * entries the app did not push, does not say where in the list the
+ * current entry is, and `popstate` fires *identically* whether the
+ * user went back or forward. So a control that wants to grey itself
+ * out has to be told, and the shell is the only thing in a position to
+ * know (#6).
+ *
+ * Two rules follow from how the shell counts, and both are the reason
+ * this is a pair of booleans rather than one depth:
+ *
+ * **Forward is not "back, negated".** `pushedEntries` -- the counter
+ * this replaces -- decremented on every `popstate`, which made a
+ * forward navigation look like a second back. The shell keeps an index
+ * per entry and a high-water mark instead, and publishes the two
+ * answers rather than the arithmetic.
+ *
+ * **Back stops at the app's own floor.** The launch entry is
+ * *replaced*, not pushed, so that one back press from the root exits
+ * the app on Android; `canBack` is false there, which is what stops
+ * the header's own button being the thing that quits.
+ */
+
+type Subscriber = () => void;
+
+export interface HistoryDepth {
+    canBack: boolean;
+    canForward: boolean;
+}
+
+class HistoryStore {
+    private depth: HistoryDepth = { canBack: false, canForward: false };
+
+    private subscribers = new Set<Subscriber>();
+
+    get(): HistoryDepth {
+        return this.depth;
+    }
+
+    /** Called by the shell whenever an entry is pushed or restored. */
+    setDepth(canBack: boolean, canForward: boolean): void {
+        if (
+            canBack === this.depth.canBack &&
+            canForward === this.depth.canForward
+        ) {
+            return;
+        }
+
+        this.depth = { canBack, canForward };
+        this.notify();
+    }
+
+    subscribe(fn: Subscriber): () => void {
+        this.subscribers.add(fn);
+
+        return () => this.subscribers.delete(fn);
+    }
+
+    private notify(): void {
+        this.subscribers.forEach((fn) => fn());
+    }
+}
+
+export const historyStore = new HistoryStore();

frontend/src/store/index.ts

@@ -11,6 +11,9 @@ export { searchStore } from './search-store';
 export { SearchController } from './controllers/search-controller';
 export { activeViewStore } from './active-view-store';
 export { ActiveViewController } from './controllers/active-view-controller';
+export { historyStore } from './history-store';
+export type { HistoryDepth } from './history-store';
+export { HistoryController } from './controllers/history-controller';
 export { shortcutsStore } from './shortcuts-store';
 export type { ShortcutsState } from './shortcuts-store';
 export { ShortcutsController } from './controllers/shortcuts-controller';