refactor(frontend): each module defines itself in its folder; rails load lazily and keep their own state

A page used to be listed three times (lib/modules for its label, icon and gate; shell/moduleRegistry for
its page and rail; lib/pageMeta for its title), and App held every rail's layout and collapse flag.
Now each module's `module.ts` says it all — label, icon, gate, its lazy page and rail, its width —
handed out through its index.ts, and features/navigation/moduleRegistry lists them in nav order for
the rail, the palette, the tab title, the reachable set and the shell's render. A contract test holds
the registry to the page ids and each definition to its own folder, with no static import of a page.

Every rail is lazy now, loaded together with its page so a pinned rail never arrives after it and
shoves the content aside; the feed's and the playlists' rails took @dnd-kit off the boot path (boot
428.5 → 393.9 KiB gzip, the App chunk 122 → 32 KiB). A rail reads its own layout and collapse flag
(lib/useRail) and writes a layout change to the `me` cache too, since it now mounts anew on each
visit. App's session effects, its one-shot entry-URL handlers and the admin nudge move into shell
hooks; App is layout. The registry's three layer suppressions are gone: none is left.

frontend/src/shell/AppCommandPalette.dom.test.tsx

@@ -26,7 +26,9 @@ vi.mock("../components/ui/CommandPalette", () => ({
 
 const channels = vi.fn();
 const playlists = vi.fn();
-vi.mock("../lib/api", () => ({
+// Partial: the module registry the palette lists reaches the feed's store, which reads its defaults here.
+vi.mock("../lib/api", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("../lib/api")>()),
   api: {
     channels: (...a: unknown[]) => channels(...a),
     playlists: (...a: unknown[]) => playlists(...a),