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/WatchPage.tsx

@@ -67,9 +67,10 @@ const STR = {
   },
 } as const;
 
-const lang = ((navigator.language || "en").slice(0, 2) as keyof typeof STR) in STR
-  ? ((navigator.language || "en").slice(0, 2) as keyof typeof STR)
-  : "en";
+const lang =
+  ((navigator.language || "en").slice(0, 2) as keyof typeof STR) in STR
+    ? ((navigator.language || "en").slice(0, 2) as keyof typeof STR)
+    : "en";
 const T = STR[lang];
 
 export default function WatchPage() {
@@ -129,7 +130,9 @@ export default function WatchPage() {
           <span className="font-semibold text-fg">Siftlode</span>
         </div>
 
-        {status === "loading" && <div className="flex-1 grid place-items-center text-muted">{T.loading}</div>}
+        {status === "loading" && (
+          <div className="flex-1 grid place-items-center text-muted">{T.loading}</div>
+        )}
 
         {status === "gone" && (
           <div className="flex-1 grid place-items-center text-center">