feat(plex): Collections Phase 2 — admin collection editing (write-back to Plex)

Admins can curate Plex collections from a movie's info page ("Collections"
button → PlexCollectionEditor dialog): create a collection (seeded with the
movie), add/remove the movie to/from editable collections, delete a collection,
and "take over" an existing plain Plex collection (mark it editable). All writes
go to Plex (POST/PUT/DELETE via new PlexClient methods) and are reflected on
every client; a targeted single-collection re-sync (sync.resync_collection /
delete_collection_local) updates the local mirror without the full ~4-min sync.

Gating (per the design decisions): editing is ADMIN-ONLY (collections are shared
library-wide); only plain manual collections are editable — smart + external
auto-lists (IMDb/TMDb/…) are always read-only (can_edit = editable && !smart &&
source=="collection"). New admin endpoints under /api/plex/collections
(create/items add+remove/rename/delete/editable). Verified end-to-end incl. the
live Plex write API (create/add/rename/remove/delete all 200, self-cleaned) and
the editor UI (create + delete with confirm) on localdev.

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

@@ -28,12 +28,28 @@
     "genre": "Genre",
     "year": "Year",
     "duration": "Length",
-    "durationOpt": { "short": "Under 90m", "mid": "90–120m", "long": "Over 2h" },
+    "durationOpt": {
+      "short": "Under 90m",
+      "mid": "90–120m",
+      "long": "Over 2h"
+    },
     "added": "Added to Plex",
-    "addedOpt": { "24h": "24h", "1w": "1 week", "1m": "1 month", "6m": "6 months", "1y": "1 year" },
+    "addedOpt": {
+      "24h": "24h",
+      "1w": "1 week",
+      "1m": "1 month",
+      "6m": "6 months",
+      "1y": "1 year"
+    },
     "contentRating": "Age rating",
-    "match": { "any": "Any", "all": "All" },
-    "dir": { "desc": "↓ Descending", "asc": "↑ Ascending" },
+    "match": {
+      "any": "Any",
+      "all": "All"
+    },
+    "dir": {
+      "desc": "↓ Descending",
+      "asc": "↑ Ascending"
+    },
     "collection": "Collection",
     "collectionSearch": "Search collections…",
     "collectionEmpty": "No collections"
@@ -59,6 +75,20 @@
     "count": "{{count}} titles",
     "added": "added"
   },
+  "collEditor": {
+    "manage": "Collections",
+    "title": "Collections — {{title}}",
+    "newPlaceholder": "New collection name…",
+    "create": "Create",
+    "search": "Filter collections…",
+    "none": "No editable collections yet. Create one above.",
+    "count": "{{count}} items",
+    "delete": "Delete",
+    "deleteConfirm": "Delete the collection \"{{title}}\" from Plex? This affects all users.",
+    "note": "Changes are written to Plex and shared across all clients and users.",
+    "others": "Manage {{count}} existing collections…",
+    "makeEditable": "Manage"
+  },
   "player": {
     "loading": "Loading…",
     "back": "Back",