feat: 4K downloads, watch progress, player controls, new icon

Window chrome: titleBarStyle Overlay (Transparent left the native bar in
place, white in dark mode, with a dead strip beneath it). The app now
paints that strip itself, so it matches the page background.

Player: prev/next through the feed, a full-screen button, a spinner
while a stream resolves, Open on YouTube on downloaded videos too, and
the description collapsed behind a disclosure.

Downloads default to Best, which reaches real 4K — above 1080p YouTube
serves VP9/AV1, verified to play natively in WKWebView here. Audio stays
pinned to AAC because Opus in MP4 would be silent. A Compatible setting
keeps the old 1080p H.264 behaviour. Files are now named
'<ISO date> - <title> [<id>].mp4'.

Watch progress is recorded and drawn under thumbnails like YouTube's,
and reopening a video resumes where it left off.

Tiles clamp every text line to a fixed height so they share a baseline,
and the search field no longer clips its placeholder.

Fixes a Picture-in-Picture leak: WebKit kept a detached video playing
after the player closed, so a second video could play over the first
with no way to stop it. Every exit path now tears the element down.

src/components/ui.tsx

@@ -43,6 +43,16 @@ export const BTN_QUIET =
   "text-[11px] text-slate-500 underline underline-offset-2 " +
   "hover:text-sky-600 dark:text-slate-400 dark:hover:text-sky-400";
 
+/** The only spinning thing in the app; used where a wait has no known length. */
+export function Spinner({ className = "size-4" }: { className?: string }) {
+  return (
+    <svg className={`${className} animate-spin`} viewBox="0 0 24 24" fill="none" aria-hidden>
+      <circle cx="12" cy="12" r="9" stroke="currentColor" strokeWidth="2.5" className="opacity-25" />
+      <path d="M21 12a9 9 0 0 0-9-9" stroke="currentColor" strokeWidth="2.5" strokeLinecap="round" />
+    </svg>
+  );
+}
+
 export function SectionHeading({
   step,
   children,