feat(v2): T-309 test-connection button with result popup

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>

v2/cmd/silver-pose/main.go

@@ -187,8 +187,9 @@ func settingsFor(cfg config.Config, configPath string) ui.Settings {
 		EventSummary: fmt.Sprintf("确认 %.1f 秒；水平角 %.0f°；快速下移=%t；下肢=%t",
 			cfg.Event.ConfirmWindowSeconds, cfg.Event.HorizontalAngleThresholdDegrees,
 			cfg.Event.RequireRapidDrop, cfg.Event.RequireLowerBody),
-		ConfigPath: configPath,
-		Camera:     cameraFieldsFrom(configPath),
+		FFprobePath: cfg.Tools.FFprobePath,
+		ConfigPath:  configPath,
+		Camera:      cameraFieldsFrom(configPath),
 		Params: ui.ParamFields{
 			KeypointConfidence:     float64(cfg.Event.KeypointConfidenceThreshold),
 			ConfirmWindowSeconds:   cfg.Event.ConfirmWindowSeconds,