feat: fullscreen carries across videos, and stays out of the way

In fullscreen, Next now keeps you there and shows nothing but the
picture: one video after another, no arrows, no header, no counter.

Three things had to change for that. The player was keyed on the video,
so every Next rebuilt the stage — and the stage is what is fullscreen.
It now survives, resetting its own per-video state instead of being
thrown away to get it.

The cleanup that ran on every change of source called exitFullscreen
outright. It exists for the Picture-in-Picture leak, which does need
handling per video; leaving fullscreen and dropping the source belong
to leaving the player, and are now only done there.

And resolving a stream set the source to nothing first, which unmounted
the element mid-fullscreen. The outgoing video is paused in place
instead, and the source goes straight from one address to the next.

The edge arrows are gone in fullscreen. The transport bar stays — it is
playback, not navigation — and fades on its own as before.

Also: Remove all downloads in the menu bar panel, which asks in the
window rather than in a panel that closes when you look away; and the
JavaScript round-trip logging is gone now that the scrape is settled.

Verified in the running app: three videos in a row without leaving
fullscreen, chrome faded to nothing, and Remove all reaching the
window's own "Delete every download?" with 50 downloads left untouched.

src/components/TrayPanel.tsx

@@ -122,6 +122,29 @@ export default function TrayPanel() {
         }
       />
 
+      <Row
+        label="Remove all downloads"
+        hint="Asks in the window first"
+        busy={busy === "wipe"}
+        onClick={() =>
+          run(
+            "wipe",
+            async () => {
+              await emitTo("main", "downloads:wipe-request", null);
+              await showMainWindow();
+            },
+            false,
+          )
+        }
+        icon={
+          <svg viewBox="0 0 24 24" className={ICON} {...stroke} strokeLinecap="round" strokeLinejoin="round">
+            <path d="M4 7h16M10 11v6M14 11v6" />
+            <path d="M6 7l1 12.5A1.5 1.5 0 008.5 21h7a1.5 1.5 0 001.5-1.5L18 7" />
+            <path d="M9 7V5a1 1 0 011-1h4a1 1 0 011 1v2" />
+          </svg>
+        }
+      />
+
       <div className="my-1 h-px bg-slate-200 dark:bg-slate-800" />
 
       <Row