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/features/navigation/NavigationProvider.tsx

@@ -4,7 +4,6 @@ import { useTranslation } from "react-i18next";
 import { useMe } from "../../lib/useMe";
 import { resolvePage, retiredParams, urlWithParams, type Page } from "../../lib/urlState";
 import { ACTIVITY_KEYS } from "../../lib/activityUrlState";
-import { pageTitleKey } from "../../lib/pageMeta";
 import { useDocumentObjectTitle } from "../../lib/documentTitle";
 import { getAccountRaw, setAccountRaw } from "../../lib/storage";
 import {
@@ -14,7 +13,7 @@ import {
   notifyNav,
   rememberNavPosition,
 } from "../../lib/history";
-import { reachablePage } from "../../lib/modules";
+import { pageTitleKey, reachablePage } from "./moduleRegistry";
 import {
   channelViewFrom,
   loadInitialPage,