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/DownloadButton.tsx

@@ -55,7 +55,7 @@ export default function DownloadButton({
         title={label}
         className={clsx(
           className ?? "p-1.5 rounded-md hover:bg-surface text-muted hover:text-fg",
-          (downloaded || inQueue) && "text-accent"
+          (downloaded || inQueue) && "text-accent",
         )}
       >
         <Download className={clsx("w-4 h-4", inQueue && "animate-pulse")} />