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/About.tsx

@@ -24,7 +24,10 @@ export default function About({
   const { data } = useQuery({ queryKey: ["version"], queryFn: api.version, staleTime: 5 * 60_000 });
 
   const rows: [string, string][] = [
-    [t("about.frontend"), FRONTEND_VERSION + (FRONTEND_SHA !== "unknown" ? ` · ${FRONTEND_SHA}` : "")],
+    [
+      t("about.frontend"),
+      FRONTEND_VERSION + (FRONTEND_SHA !== "unknown" ? ` · ${FRONTEND_SHA}` : ""),
+    ],
     [
       t("about.backend"),
       data ? data.app_version + (data.git_sha !== "unknown" ? ` · ${data.git_sha}` : "") : "…",