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/UndoToolbar.tsx

@@ -22,12 +22,7 @@ export default function UndoToolbar({
     function onKey(e: KeyboardEvent) {
       if (!(e.ctrlKey || e.metaKey)) return;
       const tgt = e.target as HTMLElement | null;
-      if (
-        tgt &&
-        (tgt.tagName === "INPUT" ||
-          tgt.tagName === "TEXTAREA" ||
-          tgt.isContentEditable)
-      )
+      if (tgt && (tgt.tagName === "INPUT" || tgt.tagName === "TEXTAREA" || tgt.isContentEditable))
         return;
       const k = e.key.toLowerCase();
       if (k === "z" && !e.shiftKey) {
@@ -50,10 +45,22 @@ export default function UndoToolbar({
     "p-1.5 rounded-lg border border-border text-muted enabled:hover:text-fg enabled:hover:border-accent disabled:opacity-30 transition";
   return (
     <div className="inline-flex items-center gap-1">
-      <button onClick={onUndo} disabled={!canUndo} title={t("common.undo")} aria-label={t("common.undo")} className={btn}>
+      <button
+        onClick={onUndo}
+        disabled={!canUndo}
+        title={t("common.undo")}
+        aria-label={t("common.undo")}
+        className={btn}
+      >
         <Undo2 className="w-4 h-4" />
       </button>
-      <button onClick={onRedo} disabled={!canRedo} title={t("common.redo")} aria-label={t("common.redo")} className={btn}>
+      <button
+        onClick={onRedo}
+        disabled={!canRedo}
+        title={t("common.redo")}
+        aria-label={t("common.redo")}
+        className={btn}
+      >
         <Redo2 className="w-4 h-4" />
       </button>
     </div>