feat(duel): allow swapping main view to opponent's board

- Add a toggle on the picture-in-picture window to swap the big board
  with the mini view of the opponent's field
- Color-coded frame and badge identify whose board is currently shown
  (blue = player 1, orange = player 2)
- Block building and rushing while spectating the opponent's board;
  the PiP then shows your own field instead
- Clarify duel-mode descriptions in lobby and start screen, add hints
  in the multiplayer bar
- Replace previous announcement banners so only one shows at a time

src/main.ts

@@ -1,5 +1,12 @@
 import { createApp } from 'vue'
 import App from './App.vue'
+import { store } from '@/game/store'
+import { mpgame } from '@/game/mpgame'
 import './style.css'
 
+// diagnostic hook (read-only debugging from devtools/automation)
+if (typeof window !== 'undefined') {
+  ;(window as unknown as Record<string, unknown>).__trxtd = { store, mpgame }
+}
+
 createApp(App).mount('#app')