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/PlexViewToggle.tsx

@@ -8,7 +8,9 @@ import { usePlexViewControls } from "./PlexViewBridge";
 export default function PlexViewToggle() {
   const { t } = useTranslation();
   const ctl = usePlexViewControls();
-  if (!ctl || !ctl.toggleable) return null;
+  // Only the two title-browse views host the Discover/Browse choice; deeper views (show/info/person/…)
+  // and the playlists/collections browse surfaces hide it.
+  if (!ctl || (ctl.kind !== "discover" && ctl.kind !== "grid")) return null;
   return (
     <SegmentedToggle
       size="md"
@@ -17,7 +19,7 @@ export default function PlexViewToggle() {
         { value: "discover", label: t("plex.discover.tab") },
         { value: "grid", label: t("plex.view.browse") },
       ]}
-      value={ctl.isDiscover ? "discover" : "grid"}
+      value={ctl.kind === "discover" ? "discover" : "grid"}
       onChange={(v) => ctl.open(v === "discover" ? { kind: "discover" } : { kind: "grid" })}
     />
   );