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/de/plex.json

@@ -28,12 +28,28 @@
     "genre": "Genre",
     "year": "Jahr",
     "duration": "Länge",
-    "durationOpt": { "short": "Unter 90 Min.", "mid": "90–120 Min.", "long": "Über 2 Std." },
+    "durationOpt": {
+      "short": "Unter 90 Min.",
+      "mid": "90–120 Min.",
+      "long": "Über 2 Std."
+    },
     "added": "Zu Plex hinzugefügt",
-    "addedOpt": { "24h": "24 Std.", "1w": "1 Woche", "1m": "1 Monat", "6m": "6 Monate", "1y": "1 Jahr" },
+    "addedOpt": {
+      "24h": "24 Std.",
+      "1w": "1 Woche",
+      "1m": "1 Monat",
+      "6m": "6 Monate",
+      "1y": "1 Jahr"
+    },
     "contentRating": "Altersfreigabe",
-    "match": { "any": "Beliebig", "all": "Alle" },
-    "dir": { "desc": "↓ Absteigend", "asc": "↑ Aufsteigend" },
+    "match": {
+      "any": "Beliebig",
+      "all": "Alle"
+    },
+    "dir": {
+      "desc": "↓ Absteigend",
+      "asc": "↑ Aufsteigend"
+    },
     "collection": "Sammlung",
     "collectionSearch": "Sammlungen suchen…",
     "collectionEmpty": "Keine Sammlungen"
@@ -59,6 +75,20 @@
     "count": "{{count}} Titel",
     "added": "hinzugefügt"
   },
+  "collEditor": {
+    "manage": "Sammlungen",
+    "title": "Sammlungen — {{title}}",
+    "newPlaceholder": "Name der neuen Sammlung…",
+    "create": "Erstellen",
+    "search": "Sammlungen filtern…",
+    "none": "Noch keine bearbeitbaren Sammlungen. Erstelle oben eine.",
+    "count": "{{count}} Titel",
+    "delete": "Löschen",
+    "deleteConfirm": "Sammlung „{{title}}“ aus Plex löschen? Betrifft alle Benutzer.",
+    "note": "Änderungen werden in Plex geschrieben und für alle Clients/Benutzer sichtbar.",
+    "others": "{{count}} vorhandene Sammlungen verwalten…",
+    "makeEditable": "Verwalten"
+  },
   "player": {
     "loading": "Wird geladen…",
     "back": "Zurück",