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/windows/notify_unsupported.go

@@ -1,10 +1,10 @@
-//go:build !windows
-
-package windows
-
-import "fyne.io/fyne/v2"
-
-func SendNotification(n *fyne.Notification, iconFilePath string) {
-	fyne.LogError("windows.SendNotification should not be invoked on non-Windows platform", nil)
-	fyne.CurrentApp().SendNotification(n)
-}
+//go:build !windows
+
+package windows
+
+import "fyne.io/fyne/v2"
+
+func SendNotification(n *fyne.Notification, iconFilePath string) {
+	fyne.LogError("windows.SendNotification should not be invoked on non-Windows platform", nil)
+	fyne.CurrentApp().SendNotification(n)
+}