fix(keyring): use platform-native keychain backends per target OS (#5)

   keyring v3 defaults to a mock file-based backend. Enable apple-native
   on macOS, windows-native on Windows, linux-native on Linux via
   platform-specific Cargo dependencies so keys actually persist in the
   system keychain.

chore: add platform-specific keyring features and simplify feature flags

Consolidate language and storage feature flags into
'all-languages' and 'secure-storage' groups to reduce verbosity.

Author: Sephyi