- 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
Upgrade to 1.21 as 1.20 is now unmaintained.
Most changes are related to the new `slices` package from standard
library, which can replace some existing custom functions.
Gianluca Boiano