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/render.ts

@@ -1,4 +1,4 @@
-import { ENEMIES, H, TILE, TOWERS, W } from './config'
+import { ENEMIES, H, TILE, TOWERS, W, playerColor } from './config'
 import type { GameEngine } from './engine'
 import type { Enemy, Tower } from './types'
 import { mulberry32 } from './utils'
@@ -342,7 +342,7 @@ export class Renderer {
     g.lineTo(x + 10, y + 26)
     g.closePath()
     g.fill()
-    // banner
+    // banner carries the owner's color in duel mode (blue/orange)
     g.strokeStyle = '#5b4a35'
     g.lineWidth = 3
     g.beginPath()
@@ -350,7 +350,7 @@ export class Renderer {
     g.lineTo(x, y - 62)
     g.stroke()
     const wave = Math.sin(eng.time * 4) * 3
-    g.fillStyle = '#d84b3f'
+    g.fillStyle = eng.mpMode === 'duel' ? playerColor(eng.localPlayerId) : '#d84b3f'
     g.beginPath()
     g.moveTo(x + 1, y - 62)
     g.lineTo(x + 22 + wave, y - 56)