Add Windows support via a Tauri (WebView2) shell

zero-native has no Windows target yet, so add a parallel Tauri shell
for Windows that reuses the same frontend:

- src-tauri/: Rust commands (yt_search, yt_streams, store_load,
  store_save) mirroring the Zig bridge; spawns bundled yt-dlp.exe with
  CREATE_NO_WINDOW; data stored in app_data_dir
- frontend bridge now auto-detects window.__TAURI__ vs window.zero and
  routes accordingly; CSP updated for Tauri IPC
- tauri.conf.json bundles yt-dlp.exe as a resource and builds nsis/msi
- scripts/make-icon.js generates appicon.png for 'tauri icon'
- README: native Windows build steps + WSLg fallback note

frontend/index.html

@@ -5,7 +5,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <meta
       http-equiv="Content-Security-Policy"
-      content="default-src 'self'; img-src 'self' https: data:; media-src https: blob:; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; font-src https://fonts.gstatic.com data:; script-src 'self'; connect-src 'self' https:;"
+      content="default-src 'self'; img-src 'self' https: data: asset: http://asset.localhost; media-src 'self' https: blob:; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; font-src https://fonts.gstatic.com data:; script-src 'self'; connect-src 'self' https: ipc: http://ipc.localhost;"
     />
     <title>YT Player</title>
     <link rel="preconnect" href="https://fonts.googleapis.com" />