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/components/PlexInfo.tsx

@@ -5,6 +5,7 @@ import { useDismiss } from "../lib/useDismiss";
 import {
   Check,
   ExternalLink,
+  FolderPlus,
   Play,
   RotateCcw,
   SlidersHorizontal,
@@ -12,6 +13,7 @@ import {
   X,
 } from "lucide-react";
 import { api, type PlexFilters, type PlexItemDetail } from "../lib/api";
+import PlexCollectionEditor from "./PlexCollectionEditor";
 
 // The rich "media info" surface for a Plex item — poster/art, IMDb score + link, content rating,
 // genres, director, cast (with photos), summary. Reused in TWO places: a full-page view opened from
@@ -30,6 +32,8 @@ type Props = {
   onFilter?: (patch: Partial<PlexFilters>) => void;
   // Play a sibling title from a collection strip (opens its player).
   onPlayItem?: (id: string) => void;
+  // The item's library key — enables the admin "Manage collections" editor (page variant, movies).
+  library?: string;
 };
 
 // A metadata value that filters the grid when clicked (if onFilter is wired), else plain text.
@@ -67,6 +71,7 @@ export default function PlexInfo({
   onStateChange,
   onFilter,
   onPlayItem,
+  library,
 }: Props) {
   const { t } = useTranslation();
   const qc = useQueryClient();
@@ -88,6 +93,8 @@ export default function PlexInfo({
       return next;
     });
   };
+  const isAdmin = (qc.getQueryData<{ role?: string }>(["me"])?.role) === "admin";
+  const [editorOpen, setEditorOpen] = useState(false);
   const [customizing, setCustomizing] = useState(false);
   const custBtnRef = useRef<HTMLButtonElement>(null);
   const custMenuRef = useRef<HTMLDivElement>(null);
@@ -360,6 +367,15 @@ export default function PlexInfo({
                     {t("plex.info.clearResume")}
                   </button>
                 )}
+                {isAdmin && detail.kind === "movie" && library && (
+                  <button
+                    onClick={() => setEditorOpen(true)}
+                    className="glass-card glass-hover inline-flex items-center gap-1.5 rounded-xl px-3 py-2 text-sm hover:text-accent"
+                  >
+                    <FolderPlus className="w-4 h-4" />
+                    {t("plex.collEditor.manage")}
+                  </button>
+                )}
               </div>
             )}
           </div>
@@ -488,6 +504,15 @@ export default function PlexInfo({
           </div>
         ))}
       </div>
+      {editorOpen && library && (
+        <PlexCollectionEditor
+          item={{ id: detail.id, title: detail.title }}
+          library={library}
+          memberOf={(detail.collections ?? []).map((c) => c.id)}
+          onClose={() => setEditorOpen(false)}
+          onChanged={() => onStateChange?.()}
+        />
+      )}
     </div>
   );
 }