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/e2e/nav.spec.ts

@@ -7,7 +7,17 @@ import { openApp } from "./helpers";
 test.describe("page switching", () => {
   // Core + admin modules (the E2E account is an admin, matching the real primary user), so this
   // also exercises the render ternary's admin branches (scheduler / config / users / audit).
-  const RAIL = ["channels", "playlists", "stats", "scheduler", "config", "users", "audit", "settings", "feed"];
+  const RAIL = [
+    "channels",
+    "playlists",
+    "stats",
+    "scheduler",
+    "config",
+    "users",
+    "audit",
+    "settings",
+    "feed",
+  ];
 
   test("each rail item activates its page", async ({ page }) => {
     await openApp(page);