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/output/sender.go

@@ -37,6 +37,7 @@ type Sender struct {
 	port     int
 
 	dropped atomic.Int64
+	bytes   atomic.Int64
 }
 
 // New creates an unconnected Sender.
@@ -95,6 +96,10 @@ func (s *Sender) Send(b []byte) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
+	// Count the packet once as produced bandwidth (independent of how many
+	// receivers are attached).
+	s.bytes.Add(int64(len(b)))
+
 	for c, ch := range s.conns {
 		switch {
 		case len(ch) > dropAt:
@@ -125,6 +130,11 @@ func (s *Sender) Dropped() int64 {
 	return s.dropped.Load()
 }
 
+// BytesSent returns the total number of payload bytes handed to Send.
+func (s *Sender) BytesSent() int64 {
+	return s.bytes.Load()
+}
+
 // Close shuts down the listener and waits for all writer goroutines to
 // drain. After Close returns the Sender must not be reused.
 func (s *Sender) Close() {