Improve code quality and thread safety (#844)

- Format Go files (gofmt) for backend/windows/notify*.go
- Add thread safety to Stopwatch with sync.Mutex
- Add comprehensive unit tests for Stopwatch with race detection
- Add godoc comments to ImageCache public APIs
- Improve IPC documentation with platform-specific socket path details
- Document portable mode TODO for future enhancement

backend/util/stopwatch_test.go

@@ -0,0 +1,167 @@
+package util
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestStopwatch_Basic(t *testing.T) {
+	sw := &Stopwatch{}
+
+	// Test initial state
+	if elapsed := sw.Elapsed(); elapsed != 0 {
+		t.Errorf("Expected initial elapsed time to be 0, got %v", elapsed)
+	}
+
+	// Test start and elapsed
+	sw.Start()
+	time.Sleep(10 * time.Millisecond)
+	elapsed := sw.Elapsed()
+	if elapsed < 10*time.Millisecond {
+		t.Errorf("Expected at least 10ms elapsed, got %v", elapsed)
+	}
+
+	// Test stop
+	sw.Stop()
+	stoppedElapsed := sw.Elapsed()
+	time.Sleep(10 * time.Millisecond)
+	if sw.Elapsed() != stoppedElapsed {
+		t.Error("Elapsed time should not increase after Stop()")
+	}
+
+	// Test reset
+	sw.Reset()
+	if elapsed := sw.Elapsed(); elapsed != 0 {
+		t.Errorf("Expected elapsed time to be 0 after reset, got %v", elapsed)
+	}
+}
+
+func TestStopwatch_StartStop(t *testing.T) {
+	sw := &Stopwatch{}
+
+	// Start, accumulate some time
+	sw.Start()
+	time.Sleep(10 * time.Millisecond)
+	sw.Stop()
+	firstElapsed := sw.Elapsed()
+
+	// Start again, accumulate more time
+	sw.Start()
+	time.Sleep(10 * time.Millisecond)
+	sw.Stop()
+	secondElapsed := sw.Elapsed()
+
+	if secondElapsed <= firstElapsed {
+		t.Errorf("Expected elapsed time to accumulate, first=%v second=%v", firstElapsed, secondElapsed)
+	}
+}
+
+func TestStopwatch_DoubleStart(t *testing.T) {
+	sw := &Stopwatch{}
+
+	sw.Start()
+	time.Sleep(5 * time.Millisecond)
+	firstStart := sw.Elapsed()
+
+	// Second Start() should be no-op
+	sw.Start()
+	time.Sleep(5 * time.Millisecond)
+	secondStart := sw.Elapsed()
+
+	// Time should continue from first start
+	if secondStart < firstStart {
+		t.Error("Second Start() affected timing")
+	}
+}
+
+func TestStopwatch_DoubleStop(t *testing.T) {
+	sw := &Stopwatch{}
+
+	sw.Start()
+	time.Sleep(10 * time.Millisecond)
+	sw.Stop()
+	elapsed := sw.Elapsed()
+
+	// Second Stop() should be no-op
+	sw.Stop()
+	if sw.Elapsed() != elapsed {
+		t.Error("Second Stop() changed elapsed time")
+	}
+}
+
+func TestStopwatch_ConcurrentAccess(t *testing.T) {
+	sw := &Stopwatch{}
+	var wg sync.WaitGroup
+
+	// Test concurrent Start/Stop/Elapsed calls
+	// This should not cause data races
+	const goroutines = 10
+	const iterations = 100
+
+	wg.Add(goroutines * 3)
+
+	// Concurrent starts
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			for j := 0; j < iterations; j++ {
+				sw.Start()
+				time.Sleep(time.Microsecond)
+			}
+		}()
+	}
+
+	// Concurrent stops
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			for j := 0; j < iterations; j++ {
+				sw.Stop()
+				time.Sleep(time.Microsecond)
+			}
+		}()
+	}
+
+	// Concurrent reads
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			for j := 0; j < iterations; j++ {
+				_ = sw.Elapsed()
+				time.Sleep(time.Microsecond)
+			}
+		}()
+	}
+
+	wg.Wait()
+
+	// If we got here without data races, the test passes
+	// Run with: go test -race
+}
+
+func TestStopwatch_Reset(t *testing.T) {
+	sw := &Stopwatch{}
+
+	// Reset when stopped
+	sw.Reset()
+	if elapsed := sw.Elapsed(); elapsed != 0 {
+		t.Errorf("Expected 0 after reset, got %v", elapsed)
+	}
+
+	// Reset when running
+	sw.Start()
+	time.Sleep(10 * time.Millisecond)
+	sw.Reset()
+	if elapsed := sw.Elapsed(); elapsed != 0 {
+		t.Errorf("Expected 0 after reset while running, got %v", elapsed)
+	}
+
+	// After reset, should be able to start again
+	sw.Start()
+	time.Sleep(5 * time.Millisecond)
+	elapsed := sw.Elapsed()
+	if elapsed < 5*time.Millisecond {
+		t.Errorf("Expected at least 5ms after reset and start, got %v", elapsed)
+	}
+}