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/PlaylistsRail.tsx

@@ -14,7 +14,11 @@ import { usePlaylists, usePlaylistsActions } from "./PlaylistsProvider";
 import { useMe } from "../lib/useMe";
 import { playlistName } from "../lib/playlistName";
 
-type PlSort = { key: "custom" | "name" | "count" | "duration"; dir: "asc" | "desc"; dirtyFirst: boolean };
+type PlSort = {
+  key: "custom" | "name" | "count" | "duration";
+  dir: "asc" | "desc";
+  dirtyFirst: boolean;
+};
 const PL_SORT_DEFAULT: PlSort = { key: "custom", dir: "asc", dirtyFirst: false };
 
 // The Playlists module's left rail — lifted to App level (like the filter rails) so it floats in the
@@ -92,9 +96,10 @@ export default function PlaylistsRail({
 
   const q = search.trim().toLowerCase();
   const visiblePlaylists = useMemo(
-    () => (q ? sortedPlaylists.filter((p) => plName(p).toLowerCase().includes(q)) : sortedPlaylists),
+    () =>
+      q ? sortedPlaylists.filter((p) => plName(p).toLowerCase().includes(q)) : sortedPlaylists,
     // eslint-disable-next-line react-hooks/exhaustive-deps
-    [sortedPlaylists, q]
+    [sortedPlaylists, q],
   );
 
   useEffect(() => {
@@ -161,7 +166,11 @@ export default function PlaylistsRail({
                 title={plSort.dir === "asc" ? t("playlists.dirAsc") : t("playlists.dirDesc")}
                 className="p-1 rounded-md border border-border text-muted enabled:hover:text-fg enabled:hover:border-accent disabled:opacity-30 transition"
               >
-                {plSort.dir === "asc" ? <ArrowUp className="w-3.5 h-3.5" /> : <ArrowDown className="w-3.5 h-3.5" />}
+                {plSort.dir === "asc" ? (
+                  <ArrowUp className="w-3.5 h-3.5" />
+                ) : (
+                  <ArrowDown className="w-3.5 h-3.5" />
+                )}
               </button>
               <button
                 onClick={() => setPlSort({ ...plSort, dirtyFirst: !plSort.dirtyFirst })}
@@ -235,7 +244,8 @@ export default function PlaylistsRail({
                   </div>
                   <div className="text-[11px] text-muted">
                     {t("playlists.itemCount", { count: pl.item_count })}
-                    {pl.total_duration_seconds > 0 && ` · ${formatDuration(pl.total_duration_seconds)}`}
+                    {pl.total_duration_seconds > 0 &&
+                      ` · ${formatDuration(pl.total_duration_seconds)}`}
                   </div>
                 </div>
               </button>