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/components/plexSubUrl.ts

@@ -14,7 +14,8 @@ export type Sub =
   | { kind: "player"; id: string; queue?: string[] }
   | { kind: "info"; id: string }
   | { kind: "person"; name: string }
-  | { kind: "playlist"; id: number };
+  | { kind: "playlist"; id: number }
+  | { kind: "collection"; id: string }; // the Collection info page (id = collection rating_key)
 
 // Mirror the Plex sub-view to ?v=<kind>:<args> so a show / season / movie-info / person / playlist page
 // — and the flat browse grid — are bookmarkable. Discover is the ROOT landing (E5 S4), so it (and the
@@ -41,6 +42,8 @@ export const PLEX_SUB_URL: SubviewUrl<Sub> = {
         return `person:${v.name}`;
       case "playlist":
         return `playlist:${v.id}`;
+      case "collection":
+        return `collection:${v.id}`;
       default:
         return null; // discover (root) + the ephemeral player are not URL-mirrored
     }
@@ -68,6 +71,8 @@ export const PLEX_SUB_URL: SubviewUrl<Sub> = {
         const id = parsePositiveIntId(arg);
         return id != null ? { kind: "playlist", id } : null;
       }
+      case "collection":
+        return arg ? { kind: "collection", id: arg } : null;
       case "season": {
         const [showId, seasonId] = arg.split(":");
         return showId && seasonId ? { kind: "season", showId, seasonId } : null;