feat: add bitrate measurement and quality presets

- sender tracks payload bytes; engine measures stream bitrate over a
  sliding window and exposes it in Status
- GUI status label and tray tooltip show the live bitrate (e.g. 36 Mbps)
- add a Preset dropdown (Low/Medium/High/Ultra) that sets quality+fps
  together and applies live via SetConfig

Verified: 36 Mbps shown for Medium (quality 70 @ 30fps) matches the
wire measurement.

internal/gui/gui.go

@@ -61,6 +61,7 @@ type App struct {
 	portEnt     *widget.Entry
 	qualitySel  *widget.Select
 	fpsSel      *widget.Select
+	presetSel   *widget.Select
 	nameEnt     *widget.Entry
 	audioChk    *widget.Check
 	announceChk *widget.Check
@@ -147,6 +148,12 @@ func (g *App) buildUI() {
 	g.fpsSel = widget.NewSelect([]string{"15", "30", "60"}, g.applyLiveSettings)
 	g.fpsSel.SetSelected(itoa(g.cfg.FPS))
 
+	// Preset: one-click quality/fps combos. Choosing one sets the Quality and
+	// Frame rate selectors and applies them (live if running). Created after
+	// the quality/fps selects so applyPreset's references are valid.
+	g.presetSel = widget.NewSelect([]string{"Low", "Medium", "High", "Ultra"}, g.applyPreset)
+	g.presetSel.SetSelected("High")
+
 	// Audio.
 	g.audioChk = widget.NewCheck("Capture system audio", nil)
 	g.audioChk.SetChecked(g.cfg.Audio)
@@ -173,6 +180,7 @@ func (g *App) buildUI() {
 			{Text: "Port", Widget: g.portEnt},
 			{Text: "Source", Widget: g.srcSel},
 			{Text: "Monitor", Widget: g.monSel},
+			{Text: "Preset", Widget: g.presetSel},
 			{Text: "Quality", Widget: g.qualitySel},
 			{Text: "Frame rate", Widget: g.fpsSel},
 			{Text: "", Widget: g.audioChk},
@@ -241,6 +249,25 @@ func (g *App) selectedMonitorIndex() int {
 	return g.cfg.StreamIndex
 }
 
+// applyPreset applies a named quality/fps preset to the Quality and Frame
+// rate selectors, then pushes it live if the engine is running.
+func (g *App) applyPreset(string) {
+	var q, f int
+	switch g.presetSel.Selected {
+	case "Low":
+		q, f = 50, 15
+	case "Medium":
+		q, f = 70, 30
+	case "Ultra":
+		q, f = 100, 60
+	default: // High
+		q, f = 85, 30
+	}
+	g.qualitySel.SetSelected(itoa(q))
+	g.fpsSel.SetSelected(itoa(f))
+	g.applyLiveSettings("")
+}
+
 // 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
@@ -354,6 +381,9 @@ func (g *App) watchStats() {
 						g.statusLab.Importance = widget.SuccessImportance
 					}
 					tip := "TeleportFling · " + frames + " frames, " + dropped + " dropped"
+					if st.Bitrate > 0 {
+						tip += " · " + formatBitrate(st.Bitrate)
+					}
 					if st.Err != nil {
 						tip += " · error"
 					}
@@ -372,12 +402,27 @@ func formatStatus(st flinger.Status) string {
 	if st.Dropped > 0 {
 		base += " · " + itoa(int(st.Dropped)) + " dropped"
 	}
+	if st.Bitrate > 0 {
+		base += " · " + formatBitrate(st.Bitrate)
+	}
 	if st.Err != nil {
 		base += "\nError: " + st.Err.Error()
 	}
 	return base
 }
 
+// formatBitrate renders a bits/sec value in a human-readable form.
+func formatBitrate(bps int64) string {
+	switch {
+	case bps >= 1_000_000:
+		return fmt.Sprintf("%.1f Mbps", float64(bps)/1_000_000)
+	case bps >= 1_000:
+		return fmt.Sprintf("%.0f kbps", float64(bps)/1_000)
+	default:
+		return fmt.Sprintf("%d bps", bps)
+	}
+}
+
 // stop halts the engine and returns the UI to the stopped state.
 func (g *App) stop() {
 	if g.statsDone != nil {