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/Tooltip.tsx

@@ -55,7 +55,7 @@ export default function Tooltip({
     const half = tip.offsetWidth / 2;
     const left = Math.min(
       Math.max(r.left + r.width / 2, half + MARGIN),
-      window.innerWidth - half - MARGIN
+      window.innerWidth - half - MARGIN,
     );
     if (Math.abs(left - coords.left) > 0.5) setCoords((c) => (c ? { ...c, left } : c));
   }, [coords]);
@@ -87,7 +87,7 @@ export default function Tooltip({
           >
             {hint}
           </div>,
-          document.body
+          document.body,
         )}
     </span>
   );