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/zh.json

@@ -334,6 +334,7 @@
   "streams.updated": "源更新成功",
   "streams.deleted": "源删除成功",
   "streams.delete.confirm": "确定要删除此源吗？",
+  "streams.modal.loading": "加载中...",
   "streams.error.load": "加载源失败",
   "streams.error.required": "请填写所有必填项",
   "streams.error.delete": "删除源失败",