feat: tile view, in-app streaming, and self-painted title bar

Clicking an undownloaded video now plays it in the app instead of
handing off to the browser. YouTube's iframe embed cannot be used —
it rejects a tauri:// origin with Error 153 — so yt-dlp resolves
YouTube's HLS master playlist instead, whose H.264+AAC variants
AVFoundation streams natively in WKWebView, adaptive up to 1080p.

Adds an optional Tiles view alongside the list, remembered between
launches.

The title bar is now transparent with the title hidden, and the app
paints that strip itself in the page background so it matches instead
of showing macOS chrome beside the traffic lights.

src/components/Sidebar.tsx

@@ -23,7 +23,7 @@ export default function Sidebar({
   return (
     <aside
       className="flex max-h-[45vh] w-full shrink-0 flex-col overflow-hidden border-b
-                 border-slate-300 bg-white lg:h-screen lg:max-h-none lg:w-[280px]
+                 border-slate-300 bg-white lg:h-full lg:max-h-none lg:w-[280px]
                  lg:border-b-0 lg:border-r dark:border-slate-800 dark:bg-slate-900"
     >
       <header