feat(udp): spot clicks switch the decoder's mode

Configure (message 15), mode field only, every other field sent as
no-change: an FT4 spot clicked while WSJT-X sits in FT8 lands the operator
ready to decode instead of staring at gibberish. Sent to every instance
heard this session — one already in the right mode treats it as a no-op —
and only for modes the decoder actually speaks (FT8/FT4/JT65/JT9/MSK144/
Q65/FST4); CW and SSB are none of its business. Toggle in the Connections
panel, on by default.

app_wsjt_highlight.go

@@ -14,7 +14,43 @@ import (
 	udp "hamlog/internal/integrations/udp"
 )
 
-const keyWsjtHighlight = "udp.wsjt.highlight"
+const (
+	keyWsjtHighlight  = "udp.wsjt.highlight"
+	keyWsjtFollowMode = "udp.wsjt.followmode" // spot clicks switch the decoder's mode
+)
+
+// wsjtModes are the modes a Configure message can meaningfully ask for — the
+// decoder's own vocabulary. Anything else (CW, SSB, RTTY) is none of its
+// business and is not sent.
+var wsjtModes = map[string]bool{
+	"FT8": true, "FT4": true, "JT65": true, "JT9": true,
+	"MSK144": true, "Q65": true, "FST4": true, "JS8": false, // JS8Call speaks another protocol
+}
+
+// GetWsjtFollowMode reports whether spot clicks retune the decoder's mode.
+func (a *App) GetWsjtFollowMode() bool {
+	return a.settingOr(keyWsjtFollowMode, "1") == "1"
+}
+
+// SetWsjtFollowMode flips it.
+func (a *App) SetWsjtFollowMode(on bool) {
+	v := "0"
+	if on {
+		v = "1"
+	}
+	a.setSetting(keyWsjtFollowMode, v)
+}
+
+// ConfigureDecoderMode asks the connected decoders to switch mode — called by
+// the frontend after a spot click has tuned the radio. A no-op for modes the
+// decoder does not speak, and when the option is off or nothing is connected.
+func (a *App) ConfigureDecoderMode(mode string) {
+	mode = strings.ToUpper(strings.TrimSpace(mode))
+	if a.udp == nil || !wsjtModes[mode] || !a.GetWsjtFollowMode() {
+		return
+	}
+	a.udp.SendConfigureMode(mode)
+}
 
 // The palette. Fixed colours, not theme tokens — they are painted into another
 // application's window, which has no idea what theme OpsLog wears.