feat(plex): Collection info page — hero, play/shuffle, and write-through editing (B)

Clicking a collection tile now opens a dedicated Collection info page (like the Plex-web
one) instead of just applying it as a filter. New GET /collections/{rk} returns the hero
aggregates (year range, most-restrictive member cert, total runtime) + the members ordered
by release year then title; a new PlexCollectionView renders the hero (art/title/meta/
summary+More) and the member grid, with a Play-all and Shuffle over the playable leaves
(reusing the player's existing queue + auto-advance).

For a non-external, admin-enabled collection the page offers rename / delete / add-title
(a search picker) / remove-item, all through the pre-existing admin endpoints that write
THROUGH to Plex (each edit awaits and re-fetches — no optimism, since a Plex write can 422).
An eligible collection shows "Enable editing" first (the DB editable flag). Read-only ones
(smart / imdb / tmdb / … auto-lists) are marked with a lock on both the info page AND the
browse tile, up front.

New `{ kind: "collection" }` sub-view wired through plexSubUrl (bookmarkable ?v=collection:rk),
PlexBrowse (render branch, Backspace back-nav, tile handler), with a round-trip test.

Verified live: tile→page, hero + chronological members (James Bond 1962→2021, PG-13, 55h),
Play/Shuffle, Enable editing → edit controls + remove-X, the add search (marks already-in
titles "Added"), and the read-only lock on external collections. Real Plex mutations wired
but not executed against the live account.

Gate: tsc/lint/knip/prettier clean, 424 vitest; backend ruff clean, 6 collections tests
(+ a detail-hero/ordering assertion).

frontend/src/lib/api/plex.ts

@@ -187,6 +187,13 @@ export interface PlexCollection {
   art?: string[]; // up to 2 poster rating_keys for the browse-tile montage (browse union only)
   total_runtime_s?: number; // total runtime of all items, for the tile sub-line (browse union only)
 }
+// The Collection info page: the card fields + hero aggregates + the ordered members.
+export interface PlexCollectionDetail extends PlexCollection {
+  content_rating?: string | null;
+  year_min?: number | null;
+  year_max?: number | null;
+  items: PlexCard[];
+}
 export interface PlexPlaylist {
   id: number;
   title: string;
@@ -417,6 +424,9 @@ export const plexApi = {
     const qs = u.toString();
     return req(`/api/plex/collections${qs ? `?${qs}` : ""}`);
   },
+  // One collection's hero + ordered members (the Collection info page).
+  plexCollection: (rk: string): Promise<PlexCollectionDetail> =>
+    req(`/api/plex/collections/${encodeURIComponent(rk)}`),
   // --- Collection editing (admin only; writes back to Plex) ---
   plexCreateCollection: (
     library: string,