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/lib/moduleDef.ts

@@ -0,0 +1,40 @@
+import type { LucideIcon } from "lucide-react";
+import type { Me } from "./api";
+import type { Page } from "./urlState";
+import type { PreloadableComponent } from "./lazyWithPreload";
+
+// What a module says about itself: one definition per page, in its own folder (`modules/<page>/module.ts`,
+// handed out through its index.ts), collected in nav order by features/navigation/moduleRegistry. A new
+// page is its folder, its id in lib/urlState, and one line in that registry.
+export type ModuleDef = {
+  id: Page;
+  // i18n key for the SHORT display name — the label the nav rail and the palette show.
+  labelKey: string;
+  // The SHORT name under the icon when the user turns the rail labels on (B7): the rail is 3.75rem
+  // wide with labels or without, so it has to fit there ("Alerts", "Listák"). The full `labelKey`
+  // stays the tooltip and the accessible name.
+  railKey: string;
+  // i18n key for the LONGER, descriptive browser-tab title (e.g. "Channel manager" vs the rail's
+  // "Channels"). Absent → the tab title falls back to `labelKey`. Read via `pageTitleKey`.
+  titleKey?: string;
+  icon: LucideIcon;
+  // Admin/system modules render in their own section (below a divider) in the nav rail.
+  system?: boolean;
+  // Carries a live unread/active count in the rail (the number itself is computed in NavSidebar,
+  // which owns the polling queries; this only flags which modules have one).
+  hasBadge?: boolean;
+  // Availability for THIS account, derived from language-independent account state (plex toggle,
+  // admin role). Absent = always available. Drives the reachable set: the rail, the palette, the
+  // `g` chords, and which page a link may land on.
+  gate?: (me: Me) => boolean;
+  // The page itself: lazy, its own chunk, fetched on the first visit or by the idle preload. Prop-free:
+  // it reads `me` and its module's state from hooks.
+  Page: PreloadableComponent<object>;
+  // Its side rail, if it has one: lazy like the page, and prop-free (its layout and collapse flag are
+  // its own, lib/useRail).
+  Rail?: PreloadableComponent<object>;
+  // The content column's max width. The header + toolbar centre on this same width (via --content-mw)
+  // so the floating chrome lines up with the centred content instead of spanning the whole column.
+  // MUST match the max-w the page's own content wrapper uses. Omit for a full-width page (feed).
+  maxWidth?: string;
+};