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/types.ts

@@ -86,4 +86,14 @@ export interface ImportPreview {
   removed_downloads: number;
 }
 
-export type Quality = "best" | "compatible";
+/** "best" or a maximum height in pixels. */
+export type Quality = "best" | "2160" | "1440" | "1080" | "720" | "480";
+
+export const QUALITIES: Array<{ value: Quality; label: string }> = [
+  { value: "best", label: "Best available (up to 4K)" },
+  { value: "2160", label: "2160p — 4K" },
+  { value: "1440", label: "1440p — 2K" },
+  { value: "1080", label: "1080p" },
+  { value: "720", label: "720p" },
+  { value: "480", label: "480p" },
+];