feat: domino gameplay UI overhaul + fix animation origin

- Add fromPlayer param to animatePlacement (top for opponent, bottom for me)
- Rewrite buildLayout with semantic .dg-* class structure
- Full CSS for opponent bar, score ribbon, controls, board area
- Opponent back-tiles visual with removal animations
- Bot thinking indicator in opponent meta area
- Remove dead legacy DOM elements
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/js/modules/domino/canvas/board.js

@@ -176,7 +176,7 @@ export class DominoBoard {
     return null;
   }
 
-  animatePlacement(tile, end, callback) {
+  animatePlacement(tile, end, callback, fromPlayer = 'me') {
     const ep = end === 'left' ? this.layout.endpoints.left : this.layout.endpoints.right;
     if (!ep) { callback?.(); return; }
 
@@ -194,20 +194,21 @@ export class DominoBoard {
       h = dbl ? TILE_W : TILE_H;
     }
 
+    // Animate from the correct player's side
     const startX = this.width / 2;
-    const startY = this.height + 40;
-    const duration = 320;
+    const startY = fromPlayer === 'me' ? this.height + 40 : -40;
+    const duration = 350;
     const startTime = performance.now();
 
     const animate = (now) => {
       const t = Math.min((now - startTime) / duration, 1);
-      // Ease out cubic
+      // Ease out back (slight overshoot for satisfying snap)
       const ease = 1 - Math.pow(1 - t, 3);
 
       const x = startX + (ep.x - startX) * ease;
       const y = startY + (ep.y - startY) * ease;
       const scale = 1.3 + (1 - 1.3) * ease;
-      const alpha = 0.4 + 0.6 * ease;
+      const alpha = 0.3 + 0.7 * ease;
 
       this.animatingTile = { tile, x, y, w, h, rotation: 0, scale, alpha };
       this.draw();