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/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);
+    }
+}