style(frontend): Prettier config + one-time repo-wide format pass

Prettier 3 (pinned), printWidth 100, double quotes — chosen to match the existing
hand-formatting and minimise reflow. This commit is ONLY the mechanical format pass
(`prettier --write`) plus the config/ignore/scripts, isolated so it never pollutes a
feature diff. Verified behaviour-neutral: typecheck, eslint (0 errors), and vitest
all green before and after. See .git-blame-ignore-revs — `git blame` skips this sha.

frontend/src/components/PlexCollectionEditor.tsx

@@ -44,7 +44,10 @@ export default function PlexCollectionEditor({
   );
   // Existing Plex collections you could take over (plain, non-smart, non-auto) but haven't marked yet.
   const eligible = useMemo(
-    () => (listQ.data?.collections ?? []).filter((c) => !c.can_edit && !c.smart && c.source === "collection"),
+    () =>
+      (listQ.data?.collections ?? []).filter(
+        (c) => !c.can_edit && !c.smart && c.source === "collection",
+      ),
     [listQ.data],
   );
   const term = q.trim().toLowerCase();
@@ -102,7 +105,8 @@ export default function PlexCollectionEditor({
   }
 
   async function removeCollection(id: string, title: string) {
-    if (!(await confirm({ message: t("plex.collEditor.deleteConfirm", { title }), danger: true }))) return;
+    if (!(await confirm({ message: t("plex.collEditor.deleteConfirm", { title }), danger: true })))
+      return;
     setBusy(id);
     try {
       await api.plexDeleteCollection(id);
@@ -210,7 +214,10 @@ export default function PlexCollectionEditor({
             className="mt-2 max-h-40 space-y-1 overflow-y-auto no-scrollbar"
           >
             {eligible.map((c) => (
-              <div key={c.id} className="glass-card flex items-center gap-2 rounded-lg px-2 py-1.5 text-sm">
+              <div
+                key={c.id}
+                className="glass-card flex items-center gap-2 rounded-lg px-2 py-1.5 text-sm"
+              >
                 <span className="min-w-0 flex-1 truncate">{c.title}</span>
                 <span className="shrink-0 text-xs text-muted">
                   {t("plex.collEditor.count", { count: c.child_count ?? 0 })}