feat: add SSH key passphrase field and file picker for key path

- Add ssh_key_passphrase field to ConnectionConfig model with full keyring lifecycle
- Pass passphrase to russh load_secret_key for encrypted SSH keys
- Add native file picker (FolderOpen icon) next to SSH key path input
- Add password-type input for key passphrase in connection dialog
- Make connection dialog form scrollable for short windows
- Fix alignment of key path input and browse button
- Add i18n keys (en + zh-CN)

src/types/database.ts

@@ -19,6 +19,7 @@ export interface ConnectionConfig {
   ssh_user?: string;
   ssh_password?: string;
   ssh_key_path?: string;
+  ssh_key_passphrase?: string;
   ssh_expose_lan?: boolean;
   ssl?: boolean;
   connection_string?: string;