feat: live settings changes mid-stream

Add flinger.SetConfig to update FPS/quality/name/audio/announce without
restarting the stream. Fields that require a restart (source, port,
stream-index) are preserved. The video loop reads the live config each
frame so changes apply immediately, and the GUI Quality/Frame-rate
dropdowns push changes to a running engine.

Verified live: fps 12->33 and JPEG size 210KB->1.3MB on quality change.

internal/gui/gui.go

@@ -140,11 +140,11 @@ func (g *App) buildUI() {
 	g.setupMonitorPicker()
 
 	// Quality.
-	g.qualitySel = widget.NewSelect([]string{"50", "60", "70", "80", "90", "100"}, func(string) {})
+	g.qualitySel = widget.NewSelect([]string{"50", "60", "70", "80", "90", "100"}, g.applyLiveSettings)
 	g.qualitySel.SetSelected(itoa(g.cfg.Quality))
 
 	// FPS.
-	g.fpsSel = widget.NewSelect([]string{"15", "30", "60"}, func(string) {})
+	g.fpsSel = widget.NewSelect([]string{"15", "30", "60"}, g.applyLiveSettings)
 	g.fpsSel.SetSelected(itoa(g.cfg.FPS))
 
 	// Audio.
@@ -241,6 +241,40 @@ func (g *App) selectedMonitorIndex() int {
 	return g.cfg.StreamIndex
 }
 
+// applyLiveSettings pushes the current form values (quality, fps, name,
+// audio, announce) into a running engine via SetConfig so changes take
+// effect without restarting the stream. When the engine is not running it is
+// a no-op; the values are still captured on the next Start.
+func (g *App) applyLiveSettings(string) {
+	if g.eng == nil {
+		return
+	}
+	quality, _ := atoi(g.qualitySel.Selected)
+	fps, _ := atoi(g.fpsSel.Selected)
+	announce := g.announceChk.Checked
+	cfg := flinger.Config{
+		Name:        g.nameEnt.Text,
+		Port:        g.cfg.Port,
+		Quality:     quality,
+		FPS:         fps,
+		Source:      g.cfg.Source,
+		Audio:       g.audioChk.Checked,
+		StreamIndex: g.cfg.StreamIndex,
+		Announce:    announce,
+	}
+	if err := g.eng.SetConfig(cfg); err != nil {
+		log.Printf("gui: live settings: %v", err)
+		return
+	}
+	// Keep the persisted config in sync with what we just applied.
+	g.cfg.Quality = quality
+	g.cfg.FPS = fps
+	g.cfg.Audio = cfg.Audio
+	g.cfg.Name = cfg.Name
+	ann := announce
+	g.cfg.Announce = &ann
+}
+
 // toggleStream starts or stops the engine based on current UI state.
 func (g *App) toggleStream() {
 	if g.lock {