feat(plex): add Playlist/Collection browse modes to the header pill

The header library-scope pill gains two entry points — Playlist and Collection —
next to All/Movies/Shows, a second way into each area (the rail filter is the
other). Picking Playlist shows a grid of the user's playlists (a tile opens that
playlist's view); Collection shows a searchable grid of collections (a tile
filters the title grid by that collection). Pure front-end — reuses the existing
plexPlaylists / plexCollections(union) list endpoints.

Two new top-level sub-views (`playlists` / `collections`, bookmarkable as
?v=playlists / ?v=collections) rendered by PlexBrowse via the new plexBrowseLists
surfaces (hue-tiled like the Discover genre tiles). The PlexViewBridge now carries
the view `kind`; PlexScopeToggle is a 5-way that derives its active segment from
it (browse-mode when in one, else scope) and drives the two views through the
bridge — picking a scope from a browse mode returns to the title grid. The
Discover/Browse toggle hides in these modes; Backspace + the search→grid flip
cover them.

Tests: PlexScopeToggle (5-way + browse-mode drive/active), plexSubUrl round-trip
for the two new views. Gate: tsc/lint/knip/prettier + 402 vitest.

frontend/src/components/PlexViewBridge.tsx

@@ -12,8 +12,7 @@ import { type Sub } from "./plexSubUrl";
 // lose the remount and let the view go stale across navigation. Keeping it in PlexBrowse preserves the
 // reseed; the bridge only mirrors the small slice the header needs.
 export type PlexViewControls = {
-  isDiscover: boolean; // the Discover landing vs the flat Browse grid
-  toggleable: boolean; // only discover|grid host the toggle; deeper views (show/info/person/…) hide it
+  kind: Sub["kind"]; // the current top-level plex view — the header controls derive their state from it
   open: (v: Sub) => void;
 };