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/game/mpgame.ts

@@ -172,7 +172,11 @@ class MpGameController {
       remote.mpMode = 'duel'
       remote.localPlayerId = 1 - info.you
       remote.ghost = true
+      // CRITICAL: index boards by PLAYER ID so engines[from] is always the
+      // board owned by the acting player on every client
       this.engines = [engine, remote]
+      this.engines[info.you] = engine
+      this.engines[1 - info.you] = remote
       this.remoteEngine = remote
     }
 
@@ -190,13 +194,33 @@ class MpGameController {
     this.net.remoteTick = -1
     if (this.heartbeat) clearInterval(this.heartbeat)
     this.heartbeat = setInterval(() => this.net.sendProgress(this.myTick), 100)
+    store.mp.viewOpponent = false
 
     engine.announce(
       info.mode === 'coop' ? 'Coop-Modus!' : '1v1-Duell!',
-      info.mode === 'coop' ? 'Gemeinsames Gold – Teamwork!' : 'Verteidige deine Basis!',
+      info.mode === 'coop'
+        ? 'Gemeinsames Gold – Teamwork!'
+        : 'Dies ist DEIN Feld – Gegner live im Mini-Fenster',
     )
   }
 
+  /** duel: is the opponent's board currently shown on the main canvas? */
+  get viewSwap(): boolean {
+    return store.mp.viewOpponent && this.mode === 'duel' && this.remoteEngine !== null
+  }
+
+  /** engine whose board is rendered on the big canvas */
+  mainView(): GameEngine {
+    return this.viewSwap && this.remoteEngine ? this.remoteEngine : engine
+  }
+
+  toggleView(): void {
+    if (this.mode === 'duel' && this.remoteEngine) {
+      store.mp.viewOpponent = !store.mp.viewOpponent
+      engine.cancelMode()
+    }
+  }
+
   private disposeGame(): void {
     if (this.heartbeat) clearInterval(this.heartbeat)
     this.heartbeat = null