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_test.go

@@ -129,6 +129,63 @@ func TestValidate(t *testing.T) {
 	}
 }
 
+// TestBitrateMeasurement verifies the engine reports a non-zero bitrate once
+// it has been streaming for a bit.
+func TestBitrateMeasurement(t *testing.T) {
+	cfg := DefaultConfig()
+	cfg.Source = "pattern"
+	cfg.Port = 19759
+
+	eng, err := New(cfg)
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	eng.Start()
+	defer eng.Stop()
+
+	// Connect a receiver so packets actually flow, and wait for the bitrate
+	// window to produce a measurement.
+	conn, err := net.Dial("tcp", "127.0.0.1:"+itoa(eng.sender.Port()))
+	if err != nil {
+		t.Fatalf("dial: %v", err)
+	}
+	defer func() { _ = conn.Close() }()
+	go func() {
+		buf := make([]byte, 64*1024)
+		for {
+			if _, err := conn.Read(buf); err != nil {
+				return
+			}
+		}
+	}()
+
+	deadline := time.Now().Add(4 * time.Second)
+	for time.Now().Before(deadline) {
+		if eng.Status().Bitrate > 0 {
+			return
+		}
+		time.Sleep(200 * time.Millisecond)
+	}
+	t.Error("bitrate stayed 0 after 4s of streaming")
+}
+
+// TestFormatBitrate checks the human-readable bitrate formatting.
+func TestFormatBitrate(t *testing.T) {
+	cases := []struct {
+		bps  int64
+		want string
+	}{
+		{500, "500 bps"},
+		{5_000, "5 kbps"},
+		{5_000_000, "5.0 Mbps"},
+	}
+	for _, c := range cases {
+		if got := formatBitrate(c.bps); got != c.want {
+			t.Errorf("formatBitrate(%d) = %q, want %q", c.bps, got, c.want)
+		}
+	}
+}
+
 func itoa(v int) string {
 	if v == 0 {
 		return "0"