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/plexSubUrl.ts

@@ -7,6 +7,8 @@ import { parsePositiveIntId } from "../lib/parseId";
 export type Sub =
   | { kind: "discover" }
   | { kind: "grid" }
+  | { kind: "playlists" } // browse-all-playlists surface (header pill "Playlist" mode)
+  | { kind: "collections" } // browse-all-collections surface (header pill "Collection" mode)
   | { kind: "show"; id: string }
   | { kind: "season"; showId: string; seasonId: string }
   | { kind: "player"; id: string; queue?: string[] }
@@ -25,6 +27,10 @@ export const PLEX_SUB_URL: SubviewUrl<Sub> = {
     switch (v.kind) {
       case "grid":
         return "browse";
+      case "playlists":
+        return "playlists";
+      case "collections":
+        return "collections";
       case "show":
         return `show:${v.id}`;
       case "season":
@@ -46,6 +52,10 @@ export const PLEX_SUB_URL: SubviewUrl<Sub> = {
     switch (kind) {
       case "browse":
         return { kind: "grid" };
+      case "playlists":
+        return { kind: "playlists" };
+      case "collections":
+        return { kind: "collections" };
       case "show":
         return arg ? { kind: "show", id: arg } : null;
       case "info":