fix(shell): publish the active view, so both navs follow the back path

The nav components learned where the user was from the `navigate`
CustomEvent, which only the outbound path dispatches: `popstate` calls
`handleNavigate()` directly. So a back-navigation left both of them
highlighting the view just left — desktop included, at any width, on
any back across two primary views. Opening a detail view was the same
cause wearing a different symptom: `app-sidebar` guarded on its own
item list and kept its highlight, `bottom-nav` did not and lit nothing.

It cannot be fixed by re-dispatching `navigate` — `index.ts` is that
event's document listener, so that is an infinite loop, and "please go
to X" is not the statement being made. `activeViewStore` is the shell
saying "the active view is now X", once per navigation, `popstate`
included; both navs read it through a controller and hold no
`activeView` of their own.

A store rather than an event because a component that mounts *after* a
navigation still has to know: `bottom-nav`'s drawer builds its
`app-sidebar` on open, and that copy had heard nothing at all, so the
drawer opened on Home from any page in the app.

Closes #72

frontend/src/store/active-view-store.ts

@@ -0,0 +1,90 @@
+/**
+ * Which primary view the app is showing.
+ *
+ * The shell has always known this -- `handleNavigate()` sets
+ * `#main-content`'s `data-active-view` on every path, `_isBack`
+ * included -- and never told anyone. The nav components learned it
+ * from the `navigate` CustomEvent instead, which only the *outbound*
+ * path dispatches: the `popstate` listener calls `handleNavigate()`
+ * directly. So both navs kept highlighting the view you had just left
+ * (#72).
+ *
+ * The fix cannot be a re-dispatch of `navigate`. `index.ts` is itself a
+ * document listener for it, so emitting one from inside
+ * `handleNavigate` is an infinite loop -- and the two statements are
+ * different anyway: `navigate` means *please go to X*, and 28 call
+ * sites across 18 files say it. This says *the active view is now X*,
+ * which only the shell is in a position to say and only once per
+ * navigation.
+ *
+ * Three things about it are load-bearing.
+ *
+ * **It is a store rather than an event**, because a component that
+ * mounts *after* a navigation still has to know. `bottom-nav`'s "More"
+ * drawer creates its `<app-sidebar>` on open, and that copy had heard
+ * no `navigate` at all: standing on Albums, the drawer highlighted
+ * Home -- its `activeView` default, which existed to match the landing
+ * view and matched nothing else ever after. An event has no answer for
+ * a listener that was not there; a value does.
+ *
+ * **A detail view is not a view here.** Opening one leaves the primary
+ * view it was opened from lit, which is what #72 asks for and what
+ * `app-sidebar` used to do by accident -- it guarded on
+ * `navItems.some(...)`, so a name matching no item left its highlight
+ * alone. `bottom-nav` had no such guard and so lit nothing on a detail
+ * view. Neither was correct; the sidebar was stale-but-lucky, and
+ * stating the rule once is what makes the two agree.
+ *
+ * **Whether a view is primary is the shell's fact, not this store's.**
+ * `VIEW_TAGS` in `index.ts` is the list, and a copy of it here is a
+ * second list to forget -- so the caller passes the answer it already
+ * has rather than this file re-deriving it.
+ */
+
+type Subscriber = () => void;
+
+class ActiveViewStore {
+    /** Empty until the shell's first navigation, which happens at
+     *  startup from `GetDefaultPage()`. Nothing is highlighted for that
+     *  moment, which is honest: the alternative is a written-down
+     *  default that is right only when the default page agrees with it. */
+    private activeView = '';
+
+    private subscribers = new Set<Subscriber>();
+
+    /** The active primary view, e.g. `albums`. */
+    get(): string {
+        return this.activeView;
+    }
+
+    isActive(view: string): boolean {
+        return this.activeView !== '' && this.activeView === view;
+    }
+
+    /**
+     * Called by the shell on every navigation, `popstate` included.
+     *
+     * `isPrimary` is `view in VIEW_TAGS` at the call site: a detail
+     * view reports itself and deliberately changes nothing, so the view
+     * it was opened from stays lit until the user picks another one.
+     */
+    setView(view: string, isPrimary: boolean): void {
+        if (!isPrimary) return;
+        if (view === this.activeView) return;
+
+        this.activeView = view;
+        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 activeViewStore = new ActiveViewStore();

frontend/src/store/controllers/active-view-controller.ts

@@ -0,0 +1,59 @@
+import type {
+    ReactiveController,
+    ReactiveControllerHost,
+} from 'lit';
+import { activeViewStore } from '../active-view-store';
+
+/**
+ * ActiveViewController connects a Lit component to the
+ * ActiveViewStore.
+ *
+ * Usage in a component:
+ *
+ *   private activeCtrl = new ActiveViewController(this);
+ *
+ *   render() {
+ *     const lit = this.activeCtrl.isActive('albums');
+ *   }
+ *
+ * It reads through to the store rather than copying the value into a
+ * `@state()` field, which is the point of #72: two components holding
+ * their own idea of the active view is what let them disagree with the
+ * shell and with each other.
+ */
+export class ActiveViewController implements ReactiveController {
+    private host: ReactiveControllerHost;
+    private unsubscribe?: () => void;
+
+    constructor(host: ReactiveControllerHost) {
+        this.host = host;
+        host.addController(this);
+    }
+
+    // ===============================================================
+    // LIFECYCLE HOOKS
+    // ===============================================================
+
+    hostConnected(): void {
+        this.unsubscribe = activeViewStore.subscribe(() => {
+            this.host.requestUpdate();
+        });
+    }
+
+    hostDisconnected(): void {
+        this.unsubscribe?.();
+    }
+
+    // ===============================================================
+    // DATA ACCESS
+    // ===============================================================
+
+    /** The active primary view, e.g. `albums`. */
+    get current(): string {
+        return activeViewStore.get();
+    }
+
+    isActive(view: string): boolean {
+        return activeViewStore.isActive(view);
+    }
+}

frontend/src/store/index.ts

@@ -9,6 +9,8 @@ export type { ThemeState, BackgroundShade } from './theme-store';
 export { ThemeController } from './controllers/theme-controller';
 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 { shortcutsStore } from './shortcuts-store';
 export type { ShortcutsState } from './shortcuts-store';
 export { ShortcutsController } from './controllers/shortcuts-controller';