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/templates/modals/stream.html

@@ -6,6 +6,11 @@
             <button class="modal-close-btn" onclick="closeStreamModal()" title="Close" data-i18n-aria-label="aria.close">&#x2715;</button>
         </div>
         <div class="modal-body">
+            <!-- Loading overlay shown while dropdowns are being populated -->
+            <div id="stream-modal-loading" class="modal-body-loading" style="display:none">
+                <div class="loading-spinner"></div>
+                <div class="modal-body-loading-text" data-i18n="streams.modal.loading">Loading...</div>
+            </div>
             <input type="hidden" id="stream-id">
             <form id="stream-form">
                 <div class="form-group">