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/locales/en.json

@@ -334,6 +334,7 @@
   "streams.updated": "Source updated successfully",
   "streams.deleted": "Source deleted successfully",
   "streams.delete.confirm": "Are you sure you want to delete this source?",
+  "streams.modal.loading": "Loading...",
   "streams.error.load": "Failed to load sources",
   "streams.error.required": "Please fill in all required fields",
   "streams.error.delete": "Failed to delete source",