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.test.ts

@@ -6,6 +6,8 @@ import { PLEX_SUB_URL as U, type Sub } from "./plexSubUrl";
 describe("PLEX_SUB_URL encode", () => {
   it("mirrors each bookmarkable sub-view to ?v=<kind>:<args>", () => {
     expect(U.encode({ kind: "grid" })).toBe("browse"); // the flat "Browse all" grid (E5 S4)
+    expect(U.encode({ kind: "playlists" })).toBe("playlists"); // browse-all-playlists (Piece 2)
+    expect(U.encode({ kind: "collections" })).toBe("collections"); // browse-all-collections (Piece 2)
     expect(U.encode({ kind: "show", id: "s1" })).toBe("show:s1");
     expect(U.encode({ kind: "season", showId: "s1", seasonId: "se2" })).toBe("season:s1:se2");
     expect(U.encode({ kind: "info", id: "m9" })).toBe("info:m9");
@@ -22,6 +24,8 @@ describe("PLEX_SUB_URL encode", () => {
 describe("PLEX_SUB_URL round-trip", () => {
   const cases: Sub[] = [
     { kind: "grid" },
+    { kind: "playlists" },
+    { kind: "collections" },
     { kind: "show", id: "s1" },
     { kind: "season", showId: "s1", seasonId: "se2" },
     { kind: "info", id: "m9" },