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/useUndoable.ts

@@ -30,7 +30,7 @@ interface History<T> {
 
 export function useUndoable<T>(
   initial: T,
-  opts?: { onApply?: (value: T) => void; limit?: number }
+  opts?: { onApply?: (value: T) => void; limit?: number },
 ): Undoable<T> {
   const [, force] = useReducer((n: number) => n + 1, 0);
   const ref = useRef<History<T>>({ past: [], present: initial, future: [] });
@@ -48,7 +48,7 @@ export function useUndoable<T>(
       force();
       onApplyRef.current?.(next);
     },
-    [limit]
+    [limit],
   );
 
   const undo = useCallback(() => {