fix: add status message element and fix column resizing bug

This commit fixes two bugs:

1. Issue #7: Missing status message DOM element
   - Added #status-message div to index.html
   - Added CSS styling with type-based classes (info, success, error)
   - Enhanced showMessage() to apply type classes for visual styling
   - Messages auto-hide after 3 seconds with fade-in animation

2. Column resizing visual bug:
   - Changed grid-template-columns from flexible (1fr 3fr 1fr)
   - To fixed minimum widths: minmax(200px, 250px) minmax(600px, 3fr) minmax(200px, 250px)
   - Prevents columns from resizing when content changes (captured pieces, move history)
   - Maintains stable layout throughout gameplay

Tests:
- Added status-message.test.js with 10 test cases
- Added column-resize.test.js with 8 test cases
- Tests verify DOM element existence, CSS styling, auto-hide behavior, and layout stability

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

css/main.css

@@ -46,6 +46,8 @@ body {
     display: flex;
     gap: 2rem;
     font-size: 1rem;
+    flex-wrap: wrap;
+    align-items: center;
 }
 
 .game-status span {
@@ -54,10 +56,43 @@ body {
     border-radius: 4px;
 }
 
+.status-message {
+    display: none;
+    padding: 0.75rem 1rem;
+    border-radius: 4px;
+    font-weight: 500;
+    text-align: center;
+    animation: fadeIn 0.3s ease-in;
+    flex: 1 0 100%;
+}
+
+.status-message.info {
+    background-color: #d1ecf1;
+    color: #0c5460;
+    border: 1px solid #bee5eb;
+}
+
+.status-message.success {
+    background-color: #d4edda;
+    color: #155724;
+    border: 1px solid #c3e6cb;
+}
+
+.status-message.error {
+    background-color: #f8d7da;
+    color: #721c24;
+    border: 1px solid #f5c6cb;
+}
+
+@keyframes fadeIn {
+    from { opacity: 0; transform: translateY(-10px); }
+    to { opacity: 1; transform: translateY(0); }
+}
+
 .game-container {
     flex: 1;
     display: grid;
-    grid-template-columns: 1fr 3fr 1fr;
+    grid-template-columns: minmax(200px, 250px) minmax(600px, 3fr) minmax(200px, 250px);
     gap: 2rem;
     padding: 2rem;
     max-width: 1600px;