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.go

@@ -1,14 +1,26 @@
 package util
 
-import "time"
+import (
+	"sync"
+	"time"
+)
 
+// Stopwatch is a thread-safe timer for measuring elapsed time.
+// It can be started, stopped, and reset, and supports reading
+// the elapsed time while running or stopped.
 type Stopwatch struct {
+	mu      sync.Mutex
 	running bool
 	started time.Time
 	elapsed time.Duration
 }
 
+// Start begins or resumes the stopwatch.
+// If already running, this is a no-op.
 func (s *Stopwatch) Start() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
 	if s.running {
 		return
 	}
@@ -16,7 +28,12 @@ func (s *Stopwatch) Start() {
 	s.running = true
 }
 
+// Stop pauses the stopwatch and accumulates the elapsed time.
+// If already stopped, this is a no-op.
 func (s *Stopwatch) Stop() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
 	if !s.running {
 		return
 	}
@@ -24,7 +41,13 @@ func (s *Stopwatch) Stop() {
 	s.running = false
 }
 
+// Elapsed returns the total elapsed time.
+// If the stopwatch is running, includes time since last Start().
+// Safe to call concurrently with other methods.
 func (s *Stopwatch) Elapsed() time.Duration {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
 	e := s.elapsed
 	if s.running {
 		e += time.Since(s.started)
@@ -32,7 +55,11 @@ func (s *Stopwatch) Elapsed() time.Duration {
 	return e
 }
 
+// Reset stops the stopwatch and clears the elapsed time.
 func (s *Stopwatch) Reset() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
 	s.running = false
 	s.elapsed = time.Duration(0)
 }