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

.gitignore

@@ -12,6 +12,12 @@ build.zig
 build.zig.zon
 src/runner.zig
 
+# tauri / rust build artifacts
+src-tauri/target/
+src-tauri/gen/
+# generated by `npm run tauri icon ./appicon.png`
+src-tauri/icons/
+
 # node
 node_modules/