Show an update notice when a newer release is available

[tarunspandit]

Show an update notice when a newer release is available

Summary

1. Check the latest Aperture CLI release during startup without blocking endpoint activation.
2. Compare stable vX.Y.Z versions and skip development or prerelease builds.
3. Show the new version and official release URL when the installed release is older.
4. Ignore request, timeout, parsing, and validation failures so startup behavior remains unchanged.

Testing

1. go test ./...
2. go test -race ./...
3. go vet ./...
4. go build ./cmd/aperture
5. git diff --check

Closes #8.