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/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';