feat: add snake multiplayer chat as sidebar with vivid player colors

Move chat to the right of the board using a flex wrapper that stacks
vertically on narrow screens. Restore original snake player colors by
removing the desaturation filter added with the dark theme.

assets/css/input.css

@@ -101,10 +101,54 @@
 }
 .snake-wrapper:focus { outline: none; }
 
-/* Desaturate inline-styled snake body/head colors */
-.snake-cell[style*="background"] {
-  filter: saturate(0) brightness(0.85);
+
+/* Snake game area: board + chat side-by-side */
+.snake-game-area {
+  display: flex;
+  gap: 16px;
+  align-items: flex-start;
+  justify-content: center;
 }
-.snake-cell.snake-head[style] {
-  box-shadow: none !important;
+@media (max-width: 768px) {
+  .snake-game-area { flex-direction: column; align-items: center; }
 }
+
+/* Snake chat */
+.snake-chat { width: 100%; max-width: 480px; }
+.snake-game-area .snake-chat { width: 260px; max-width: none; flex-shrink: 0; }
+.snake-chat-history {
+  height: 300px;
+  overflow-y: auto;
+  background: #334;
+  border-radius: 8px 8px 0 0;
+  padding: 8px;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+.snake-chat-msg { font-size: 0.85rem; line-height: 1.3; }
+.snake-chat-input {
+  display: flex;
+  gap: 0;
+  background: #445;
+  border-radius: 0 0 8px 8px;
+  overflow: hidden;
+}
+.snake-chat-input input {
+  flex: 1;
+  padding: 6px 10px;
+  background: transparent;
+  border: none;
+  color: inherit;
+  outline: none;
+  font-size: 0.85rem;
+}
+.snake-chat-input button {
+  padding: 6px 14px;
+  background: #556;
+  border: none;
+  color: inherit;
+  cursor: pointer;
+  font-size: 0.85rem;
+}
+.snake-chat-input button:hover { background: #667; }