Speed up camera source modal with cached enumeration and instant open

Cache camera enumeration results for 30s and limit probe range using
WMI camera count on Windows. Open source modal instantly with a loading
spinner while dropdowns are populated asynchronously.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/css/components.css

@@ -246,6 +246,25 @@ input:-webkit-autofill:focus {
     to { transform: rotate(360deg); }
 }
 
+/* Modal body loading overlay — hides form while data loads */
+.modal-body-loading {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    padding: 48px 20px;
+    gap: 12px;
+}
+
+.modal-body-loading .loading-spinner {
+    padding: 0;
+}
+
+.modal-body-loading-text {
+    color: var(--text-secondary);
+    font-size: 0.9rem;
+}
+
 /* Full-page overlay spinner */
 .overlay-spinner {
     position: fixed;