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.

CLAUDE.md

@@ -75,10 +75,12 @@ cannot (an ORM write on another unit's loaded row, SQL in `text()`, a commit by
 [`backend/app/README.md`](backend/app/README.md) with the conventions.
 Frontend, by folder under `src/`: `main.tsx` provider tree → `shell/` (`App`, header, nav rail, palette,
 boot; a shared `/watch` link boots from its own entry, `watch.html` → `watchMain.tsx` → `watch/`, held
-app-free by a contract test) → `shell/moduleRegistry` route-chunks → **`modules/<page>/`** (one folder per page: the page,
-its rail, its store, its tests) → **`features/<name>/`** (what several pages share: video cards, filters,
-downloads, the player, navigation) → the **platform layer `components/ui/`** (domain-free primitives: the
-popover engine and its menus, `Modal`/`Overlay`, `Button`/`form`, `DataTable`/`VirtualGrid`, `QueryState`,
+app-free by a contract test) → **`modules/<page>/`** (one folder per page: the page, its rail, its store, its tests,
+and its `module.ts` definition — label, icon, gate, the lazy page and rail — collected in nav order by
+`features/navigation/moduleRegistry`; a new page is its folder, its id in `lib/urlState` and one line there) →
+**`features/<name>/`** (what several pages share: video cards, filters, downloads, the player, navigation) → the
+**platform layer `components/ui/`** (domain-free primitives: the popover engine and its menus, `Modal`/`Overlay`,
+`Button`/`form`, `DataTable`/`VirtualGrid`, `QueryState`,
 …) → `lib/`. The direction is one-way and **lint-enforced**, not just described (`local/layers`, with
 its own test): an import only goes down; another module is reached only through its **`index.ts`** (its
 public face — the stores, mounted at the root and read app-wide, which `features/` may read too) or