Adds a file picker next to the Private Key textarea in the credential
dialog. Users can browse for key files (.pem, .key, id_rsa, id_ed25519,
etc.) instead of copy-pasting PEM content. Uses standard FileReader API
— no additional Tauri plugins needed.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>