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/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';