feat(plex): restore Collections filter as a cross-library union picker

The searchable Collections picker was dropped from the sidebar when browsing
went unified (movies+shows across libraries) because collections are per-library
and there's no single selected library. Bring it back library-agnostic: GET
/api/plex/collections `library` is now optional — without it the endpoint unions
collections across all enabled libraries (with it, the old single-library path the
collection editor uses). The sidebar re-adds the searchable list (chip when one is
active). Its query key is ["plex-collections","union",<search>] so a search term
equal to a library plex_key can't collide with the editor's ["plex-collections",<key>],
while the shared prefix keeps editor invalidation refreshing the picker.

frontend/src/components/PlexSidebar.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { ChevronRight, Film, Layers, ListMusic, Plus, SlidersHorizontal, Tv2, X } from "lucide-react";
 import { api, EMPTY_PLEX_FILTERS, plexFilterCount, type PlexFilters } from "../lib/api";
+import { useDebounced } from "../lib/useDebounced";
 
 // The Plex module's left filter column (mirrors the feed Sidebar's shell). Movie libraries get the
 // full metadata filter set (genre / rating / year / duration / added / content rating + the
@@ -76,8 +77,21 @@ export default function PlexSidebar({
   });
   const facets = facetsQ.data;
 
-  // (Collection filtering: the active-collection chip is set from an item info page's "Browse
-  // collection"; there's no per-library picker in the unified cross-library scope.)
+  // Collections picker (searchable; library-agnostic UNION across all enabled libraries, since the
+  // unified scope has no single library). Only fetched while no collection is active (else the chip
+  // shows instead). Applying one sets the collection filter; the actual filtering already flows through
+  // /library + /facets by rating_key.
+  const [collSearch, setCollSearch] = useState("");
+  const collSearchDeb = useDebounced(collSearch.trim(), 300);
+  const collectionsQ = useQuery({
+    // "union" disambiguates from the editor's per-library ["plex-collections", <plex_key>] key (a search
+    // term equal to a plex_key would otherwise collide); the shared prefix keeps editor invalidation working.
+    queryKey: ["plex-collections", "union", collSearchDeb],
+    queryFn: () => api.plexCollections(undefined, collSearchDeb || undefined),
+    enabled: !filters.collection,
+  });
+  const collections = collectionsQ.data?.collections ?? [];
+
   const fCount = plexFilterCount(filters);
   const activeCount = fCount + (show !== "all" ? 1 : 0) + (sort !== "title" ? 1 : 0);
   const anyActive = activeCount > 0;
@@ -257,18 +271,44 @@ export default function PlexSidebar({
             </Section>
           )}
 
-          {/* Collection — active-collection chip only (set from an item's info page "Browse
-              collection"). The searchable picker is per-library, so it's omitted in the unified scope. */}
-          {filters.collection && (
-            <Section label={t("plex.filter.collection")}>
+          {/* Collection — the active one shows as a chip; otherwise a searchable picker over the union
+              of all enabled libraries' collections (also settable from an item info page's "Browse
+              collection"). */}
+          <Section label={t("plex.filter.collection")}>
+            {filters.collection ? (
               <div className="flex flex-wrap gap-1.5">
                 <RemovableChip
                   label={filters.collectionTitle || filters.collection}
                   onRemove={() => patch({ collection: null, collectionTitle: null })}
                 />
               </div>
-            </Section>
-          )}
+            ) : (
+              <div>
+                <input
+                  value={collSearch}
+                  onChange={(e) => setCollSearch(e.target.value)}
+                  placeholder={t("plex.filter.collectionSearch")}
+                  className="mb-1.5 w-full rounded-lg border border-border bg-card px-2 py-1 text-sm focus:border-accent focus:outline-none"
+                />
+                <div className="flex max-h-56 flex-col gap-0.5 overflow-y-auto">
+                  {collections.map((c) => (
+                    <button
+                      key={c.id}
+                      onClick={() => patch({ collection: c.id, collectionTitle: c.title })}
+                      className="glass-hover flex items-center gap-2 rounded-lg px-2 py-1 text-left text-sm"
+                    >
+                      <Layers className="w-3.5 h-3.5 shrink-0 text-muted" />
+                      <span className="flex-1 truncate">{c.title}</span>
+                      <span className="text-xs text-muted">{c.child_count}</span>
+                    </button>
+                  ))}
+                  {collections.length === 0 && (
+                    <span className="px-2 py-1 text-xs text-muted">{t("plex.filter.collectionEmpty")}</span>
+                  )}
+                </div>
+              </div>
+            )}
+          </Section>
 
           {/* IMDb rating min */}
           <Section label={t("plex.filter.rating")}>