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/config.ts

@@ -15,6 +15,13 @@ export const OBSTACLE_COST: Record<'tree' | 'rock', number> = {
   rock: 40,
 }
 
+/** multiplayer player identity colors (blue = player 1, orange = player 2) */
+export const PLAYER_COLORS: [string, string] = ['#4da3ff', '#ffb14d']
+
+export function playerColor(playerId: number | null): string {
+  return playerId === null || playerId === 0 ? PLAYER_COLORS[0] : PLAYER_COLORS[1]
+}
+
 /** ground path in tile coords; -1/20 are outside the grid (spawn/exit) */
 export const MAP_WAYPOINTS: [number, number][] = [
   [-1, 5],