fix: native video fullscreen, window dragging, quality picker, spinner

Enable WKWebView's elementFullscreenEnabled at startup. It is off by
default in a Tauri window, which is why the native player had no
full-screen button and why right-click 'Enter Full Screen' did nothing.
With it on, WebKit's own control appears on the video and gives the real
system fullscreen player, so the app-level workaround is gone.

Restore window dragging: data-tauri-drag-region needs
core:window:allow-start-dragging, which was missing, leaving the
title-bar strip inert.

Download quality is now a resolution picker (best/2160/1440/1080/720/
480) instead of a two-way toggle; every selector still pins AAC audio
because Opus in MP4 is silent in WebKit.

Tile titles truncate to a single line so rows stay uniform, and the
player shows a spinner while the video buffers, not just while the
stream URL resolves.

src/components/VideoTile.tsx

@@ -60,7 +60,7 @@ export default function VideoTile({
       <div className="mt-2 flex min-w-0 flex-col">
         <button onClick={onOpen} className="cursor-pointer text-left">
           <h3
-            className="line-clamp-2 h-[2.25rem] text-[13px] font-medium leading-snug"
+            className="h-[1.25rem] truncate text-[13px] font-medium leading-5"
             title={item.title}
           >
             {item.title}