Widgets tab links the external demo when config.json names one (demo_url)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/config/config.go

@@ -44,6 +44,9 @@ type Config struct {
 	TLS          string  `json:"tls,omitempty"` // "auto" → self-signed cert in the data dir; "" → plain HTTP
 	Tokens       []Token `json:"tokens,omitempty"`
 	Nodes        []Node  `json:"nodes,omitempty"`
+	// DemoURL, when set, is linked from the Widgets tab: a page outside Ham Lab that
+	// shows the sensor API being used (examples/external-demo).
+	DemoURL string `json:"demo_url,omitempty"`
 
 	path string
 	mu   sync.Mutex