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

src-tauri/capabilities/default.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "Core capabilities for the main window. App-defined commands (yt_search, yt_streams, store_load, store_save) are not gated by the ACL.",
+  "windows": ["main"],
+  "permissions": ["core:default"]
+}