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/ErrorBoundary.tsx

@@ -32,7 +32,9 @@ export default class ErrorBoundary extends Component<Props, State> {
       return (
         <div className="min-h-screen grid place-items-center text-muted p-6 text-center">
           <div>
-            <div className="text-lg font-semibold text-fg mb-1">{i18n.t("errors.boundary.title")}</div>
+            <div className="text-lg font-semibold text-fg mb-1">
+              {i18n.t("errors.boundary.title")}
+            </div>
             <div className="text-sm mb-3">{i18n.t("errors.boundary.subtitle")}</div>
             <button
               onClick={() => location.reload()}