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/flinger/flinger.go

@@ -10,6 +10,7 @@ package flinger
 
 import (
 	"errors"
+	"fmt"
 	"image"
 	"image/color"
 	"io"
@@ -92,6 +93,9 @@ type Status struct {
 	Frames  int64
 	Dropped int64
 	Conns   int
+	// Bitrate is the measured stream bandwidth in bits per second, averaged
+	// over the previous measurement window.
+	Bitrate int64
 	// Err is the most recent runtime error encountered (capture, encode,
 	// packet or audio), or nil if the stream is healthy.
 	Err error
@@ -111,9 +115,16 @@ type Engine struct {
 	start time.Time
 	stop  chan struct{}
 
-	frames  atomic.Int64
+	frames atomic.Int64
+
 	errMu   sync.RWMutex
 	lastErr error
+
+	// bitrate tracking
+	bitMu    sync.Mutex
+	bitLast  time.Time
+	bitBytes int64
+	bitrate  int64
 }
 
 // setErr records the most recent runtime error. Pass nil to clear it.
@@ -254,15 +265,48 @@ func (e *Engine) Status() Status {
 	e.errMu.RLock()
 	err := e.lastErr
 	e.errMu.RUnlock()
+
 	return Status{
 		Running: e.stop != nil,
 		Frames:  e.frames.Load(),
 		Dropped: e.sender.Dropped(),
 		Conns:   e.sender.NumConns(),
+		Bitrate: e.measureBitrate(),
 		Err:     err,
 	}
 }
 
+// bitrateWindow is the sliding window over which bitrate is averaged.
+const bitrateWindow = 2 * time.Second
+
+// measureBitrate computes the current stream bitrate (bits/sec) over a
+// sliding window. It is called from Status.
+func (e *Engine) measureBitrate() int64 {
+	e.bitMu.Lock()
+	defer e.bitMu.Unlock()
+
+	now := time.Now()
+	bytes := e.sender.BytesSent()
+
+	if e.bitLast.IsZero() {
+		e.bitLast = now
+		e.bitBytes = bytes
+		return 0
+	}
+
+	elapsed := now.Sub(e.bitLast)
+	if elapsed < bitrateWindow {
+		return e.bitrate
+	}
+
+	// Bytes accumulated since the previous sample.
+	delta := bytes - e.bitBytes
+	e.bitrate = int64(float64(delta*8) / elapsed.Seconds())
+	e.bitLast = now
+	e.bitBytes = bytes
+	return e.bitrate
+}
+
 // audioLoop reads raw PCM and emits WAVE packets. start is the shared
 // reference clock used by the video loop so audio and video timestamps stay
 // aligned on the receiver.
@@ -375,10 +419,11 @@ func (e *Engine) statsLoop() {
 		select {
 		case <-tick.C:
 			st := e.Status()
+			rate := formatBitrate(st.Bitrate)
 			if st.Dropped > 0 {
-				log.Printf("flinger: %d frames, %d dropped, %d conns", st.Frames, st.Dropped, st.Conns)
+				log.Printf("flinger: %d frames, %d dropped, %d conns, %s", st.Frames, st.Dropped, st.Conns, rate)
 			} else {
-				log.Printf("flinger: %d frames, %d conns", st.Frames, st.Conns)
+				log.Printf("flinger: %d frames, %d conns, %s", st.Frames, st.Conns, rate)
 			}
 		case <-e.stop:
 			return
@@ -386,6 +431,18 @@ func (e *Engine) statsLoop() {
 	}
 }
 
+// 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)
+	}
+}
+
 // frameSource abstracts the frame source: real capture or the test pattern.
 type frameSource interface {
 	Next() (*capture.VideoFrame, error)