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/lib/playlistView.test.ts

@@ -12,8 +12,20 @@ const v = (id: string, title: string, channel: string, secs: number | null = 60)
   ({ id, title, channel_title: channel, duration_seconds: secs }) as Video;
 
 const columns = [
-  { key: "title", header: "Title", render: () => null, sortable: true, sortValue: (x: Video) => x.title ?? "" },
-  { key: "channel", header: "Channel", render: () => null, sortable: true, sortValue: (x: Video) => x.channel_title ?? "" },
+  {
+    key: "title",
+    header: "Title",
+    render: () => null,
+    sortable: true,
+    sortValue: (x: Video) => x.title ?? "",
+  },
+  {
+    key: "channel",
+    header: "Channel",
+    render: () => null,
+    sortable: true,
+    sortValue: (x: Video) => x.channel_title ?? "",
+  },
   {
     key: "duration",
     header: "Length",