refactor(frontend): the module stores mount from the registry, and the feed and Plex each own their saved views

A module's definition names its store (feed: filters, view, saved views; Plex: browse state, saved
views; channels, files, playlists their provider), and ModuleStores mounts them at the root in
registry order instead of a hand-nested tree in main.tsx. A contract test holds that every store's
read side is in reach and that no store's static imports leave its own module, so the order cannot
matter.

Saved views were one provider that switched between the feed and Plex by the active page and reset
on each switch. The logic is now one hook (useSavedViewsStore) with an instance per module; the
shared chip, popover and modal read the views their page or rail hands down. A feed view applied
and edited now survives a visit to Plex. Plex's views are still asked for only on its page, and a
module's manage modal still closes when its page is left. The feed's URL-override check applies
only on the feed page, where its filter params live.

The Channels store's tab type moves from the page into the store's context.

frontend/src/lib/moduleDef.ts

@@ -1,3 +1,4 @@
+import type { ComponentType, ReactNode } from "react";
 import type { LucideIcon } from "lucide-react";
 import type { Me } from "./api";
 import type { Page } from "./urlState";
@@ -37,4 +38,10 @@ export type ModuleDef = {
   // 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;
+  // The module's store, if it has one: the state its page, its rail and its readers elsewhere (the
+  // palette, the notifications, the header search) share. Mounted at the root for the whole session
+  // (features/navigation/ModuleStores), so what it holds in memory survives a visit to another page.
+  // Static, unlike the page: it is on the boot path by design. It reads no other module's store, so
+  // the order the stores nest in does not matter (moduleStores.contract.test).
+  Store?: ComponentType<{ children: ReactNode }>;
 };