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/WatchBar.tsx

@@ -0,0 +1,30 @@
+import type { FeedItem } from "../types";
+
+/** Fraction of the video watched, or null if it was never opened. */
+export function watchedFraction(item: FeedItem): number | null {
+  const { position, duration } = item;
+  if (position == null || duration == null || duration <= 0) return null;
+  return Math.min(1, Math.max(0, position / duration));
+}
+
+/**
+ * The red sliver across the bottom of a thumbnail, same idea as YouTube's.
+ * Red rather than the sky accent on purpose: the accent means "the action to
+ * take" or "currently selected", and this is neither.
+ */
+export default function WatchBar({ item }: { item: FeedItem }) {
+  const f = watchedFraction(item);
+  if (f == null || f < 0.01) return null;
+  const nearlyDone = f > 0.97;
+  return (
+    <span
+      className="absolute inset-x-0 bottom-0 h-[3px] bg-slate-950/45"
+      title={nearlyDone ? "Watched" : `${Math.round(f * 100)}% watched`}
+    >
+      <span
+        className="block h-full bg-red-600"
+        style={{ width: `${Math.max(2, f * 100)}%` }}
+      />
+    </span>
+  );
+}