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/main.tsx

@@ -38,9 +38,13 @@ de.scheme ||= "midnight";
 
 const path = window.location.pathname;
 const root =
-  path === "/privacy" ? <PrivacyPolicy /> :
-  path === "/terms" ? <Terms /> :
-  path.startsWith("/watch/") ? <WatchPage /> : (
+  path === "/privacy" ? (
+    <PrivacyPolicy />
+  ) : path === "/terms" ? (
+    <Terms />
+  ) : path.startsWith("/watch/") ? (
+    <WatchPage />
+  ) : (
     <QueryClientProvider client={queryClient}>
       <ErrorBoundary>
         <ConfirmProvider>
@@ -69,5 +73,5 @@ const root =
 createRoot(document.getElementById("root")!).render(
   <React.StrictMode>
     <Suspense fallback={<div style={{ minHeight: "100vh" }} />}>{root}</Suspense>
-  </React.StrictMode>
+  </React.StrictMode>,
 );