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/SyncStatus.tsx

@@ -37,8 +37,7 @@ export default function SyncStatus({
     refetchInterval: (query) => {
       const d = query.state.data as MyStatus | undefined;
       const busy =
-        !!d &&
-        (d.sync_active || d.channels_recent_pending > 0 || d.channels_deep_pending > 0);
+        !!d && (d.sync_active || d.channels_recent_pending > 0 || d.channels_deep_pending > 0);
       return busy ? 8_000 : 30_000;
     },
     refetchIntervalInBackground: true,
@@ -101,7 +100,7 @@ export default function SyncStatus({
   // history) so the collapsed chip still tells you when to look.
   const needsAttention = data.paused || notFull > 0;
   const countsText = `${formatViews(data.my_videos)} ${t("header.sync.yours")} / ${formatViews(
-    data.total_videos
+    data.total_videos,
   )} ${t("header.sync.total")}`;
   return (
     <div className="pointer-events-auto shrink-0">
@@ -171,7 +170,7 @@ export default function SyncStatus({
               </div>
             )}
           </div>,
-          document.body
+          document.body,
         )}
     </div>
   );