Add: tauri frontend as bevy alternative

docs/CONFIG_REFERENCE.md

@@ -36,6 +36,9 @@ level = "info"
 [controls]
 hotkey_enabled = true
 recovery_hotkey = "Ctrl+Alt+P"
+
+[frontend]
+backend = "bevy"
 ```
 
 ## Notes
@@ -43,3 +46,4 @@ recovery_hotkey = "Ctrl+Alt+P"
 - `auth_token` is generated on first run if config does not exist.
 - `window.x`, `window.y`, `window.scale`, and flag fields are persisted after matching commands.
 - On Windows, `recovery_hotkey` provides click-through recovery even when the window is non-interactive.
+- `frontend.backend` selects runtime frontend implementation (`bevy` or `tauri`).