feat: Add update checker to verify new versions on GitHub

- Add /api/check-update endpoint that queries GitHub API
- Compare current commit hash with latest on dev branch
- Add check button next to version in menu
- Show spinning icon during check, green checkmark when done
- Display "Update available" link when newer version exists
- Handle rate limits and network errors gracefully

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/static/css/style.css

@@ -364,6 +364,16 @@ main {
     height: 100%;
 }
 
+/* Spin animation for update check button */
+.spin {
+    animation: spin 1s linear infinite;
+}
+
+@keyframes spin {
+    from { transform: rotate(0deg); }
+    to { transform: rotate(360deg); }
+}
+
 /* Empty State */
 .empty-state {
     text-align: center;