feat: surface runtime errors in engine status

Record the most recent runtime error (capture, encode, packet, audio)
in the engine and expose it via Status.Err. The GUI shows it in the
status label (highlighted as a danger) and in the tray tooltip, so a
denied screen-share or encoder failure is visible without reading logs.

internal/flinger/flinger.go

@@ -15,6 +15,7 @@ import (
 	"io"
 	"log"
 	"strconv"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -91,6 +92,9 @@ type Status struct {
 	Frames  int64
 	Dropped int64
 	Conns   int
+	// Err is the most recent runtime error encountered (capture, encode,
+	// packet or audio), or nil if the stream is healthy.
+	Err error
 }
 
 // Engine owns the capture, encode and send pipeline.
@@ -106,7 +110,16 @@ type Engine struct {
 	start time.Time
 	stop  chan struct{}
 
-	frames atomic.Int64
+	frames  atomic.Int64
+	errMu   sync.RWMutex
+	lastErr error
+}
+
+// setErr records the most recent runtime error. Pass nil to clear it.
+func (e *Engine) setErr(err error) {
+	e.errMu.Lock()
+	e.lastErr = err
+	e.errMu.Unlock()
 }
 
 // New creates an engine from cfg. Capture is opened eagerly so that
@@ -208,11 +221,15 @@ func (e *Engine) Stop() {
 
 // Status returns a snapshot of the running engine.
 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(),
+		Err:     err,
 	}
 }
 
@@ -237,6 +254,7 @@ func (e *Engine) audioLoop(src io.ReadCloser) {
 			ts := uint64(time.Since(e.start))
 			packet, perr := protocol.BuildWavePacket(ts, protocol.AudioFormatS16, sampleRate, speakers, int32(frames), buf[:n])
 			if perr != nil {
+				e.setErr(perr)
 				log.Printf("flinger: wave: %v", perr)
 			} else {
 				e.sender.Send(packet)
@@ -249,6 +267,7 @@ func (e *Engine) audioLoop(src io.ReadCloser) {
 			default:
 			}
 			if !errors.Is(err, io.EOF) && !errors.Is(err, io.ErrClosedPipe) {
+				e.setErr(err)
 				log.Printf("flinger: audio: %v", err)
 			}
 		}
@@ -269,6 +288,7 @@ func (e *Engine) videoLoop() {
 			default:
 			}
 			if !errors.Is(err, io.EOF) && !errors.Is(err, io.ErrClosedPipe) {
+				e.setErr(err)
 				log.Printf("flinger: capture: %v", err)
 			}
 			continue
@@ -285,6 +305,7 @@ func (e *Engine) videoLoop() {
 		ts := uint64(now.Sub(e.start))
 		buf, err := e.encoder.EncodeBGRA(frame.Pix, frame.Width, frame.Height, e.cfg.Quality)
 		if err != nil {
+			e.setErr(err)
 			log.Printf("flinger: jpeg: %v", err)
 			continue
 		}
@@ -296,6 +317,7 @@ func (e *Engine) videoLoop() {
 			buf,
 		)
 		if err != nil {
+			e.setErr(err)
 			log.Printf("flinger: packet: %v", err)
 			continue
 		}

internal/flinger/flinger_test.go

@@ -1,6 +1,7 @@
 package flinger
 
 import (
+	"errors"
 	"net"
 	"testing"
 	"time"
@@ -77,6 +78,34 @@ func TestNewRejectsBadSource(t *testing.T) {
 	}
 }
 
+// TestSetErrStatus verifies runtime errors are exposed via Status and can be
+// cleared.
+func TestSetErrStatus(t *testing.T) {
+	cfg := DefaultConfig()
+	cfg.Source = "pattern"
+	cfg.Port = 19757
+
+	eng, err := New(cfg)
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+
+	if st := eng.Status(); st.Err != nil {
+		t.Fatalf("expected no error initially, got %v", st.Err)
+	}
+
+	sentinel := errors.New("test capture failure")
+	eng.setErr(sentinel)
+	if st := eng.Status(); st.Err != sentinel {
+		t.Errorf("expected sentinel error, got %v", st.Err)
+	}
+
+	eng.setErr(nil)
+	if st := eng.Status(); st.Err != nil {
+		t.Errorf("expected cleared error, got %v", st.Err)
+	}
+}
+
 // TestValidate rejects out-of-range values.
 func TestValidate(t *testing.T) {
 	bad := []func(*Config){