Adds a 测试连接摄像头 button next to save; it builds the RTSP URL from the current form fields and runs ffprobe (8s timeout) off the UI thread, then shows a result popup (success WxH@fps, or a redacted failure). Exports config.BuildRTSPURL and source.Probe/Redact for it. Cross-compiles for Windows; config/source tests pass. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>