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/i18n/locales/en/plex.json

@@ -2,6 +2,26 @@
   "navLabel": "Plex",
   "backToFeed": "YouTube feed",
   "backToLibrary": "Back to library",
+  "collection": {
+    "playAll": "Play all",
+    "shuffle": "Shuffle",
+    "rename": "Rename",
+    "addItem": "Add title",
+    "delete": "Delete",
+    "enableEdit": "Enable editing",
+    "removeItem": "Remove from collection",
+    "readOnly": "Read-only",
+    "empty": "This collection has no items.",
+    "more": "More",
+    "less": "Less",
+    "addSearch": "Search titles to add…",
+    "addHint": "Type to search for a title to add.",
+    "addNoResults": "No matches.",
+    "add": "Add",
+    "added": "Added",
+    "deleteConfirm": "Delete the collection \"{{title}}\"? This removes it from Plex too.",
+    "editFailed": "Collection edit failed"
+  },
   "searchPlaceholder": "Search Plex…",
   "searchField": {
     "aria": "Search scope",

frontend/src/i18n/locales/hu/plex.json

@@ -2,6 +2,26 @@
   "navLabel": "Plex",
   "backToFeed": "YouTube feed",
   "backToLibrary": "Vissza a könyvtárhoz",
+  "collection": {
+    "playAll": "Összes lejátszása",
+    "shuffle": "Keverés",
+    "rename": "Átnevezés",
+    "addItem": "Cím hozzáadása",
+    "delete": "Törlés",
+    "enableEdit": "Szerkesztés engedélyezése",
+    "removeItem": "Eltávolítás a kollekcióból",
+    "readOnly": "Csak olvasható",
+    "empty": "Ennek a kollekciónak nincsenek elemei.",
+    "more": "Több",
+    "less": "Kevesebb",
+    "addSearch": "Keress címet a hozzáadáshoz…",
+    "addHint": "Gépelj a hozzáadni kívánt cím kereséséhez.",
+    "addNoResults": "Nincs találat.",
+    "add": "Hozzáad",
+    "added": "Hozzáadva",
+    "deleteConfirm": "Törlöd a(z) „{{title}}” kollekciót? Ez a Plexből is eltávolítja.",
+    "editFailed": "A kollekció szerkesztése sikertelen"
+  },
   "searchPlaceholder": "Keresés a Plexben…",
   "searchField": {
     "aria": "Keresési hatókör",