feat(security): production-ready SSH tunneling and Bastion Host support

[ZhuchkaTriplesix]

Summary

Implements built-in SSH Bastion / Jump Host tunneling support for PostgreSQL, MySQL, MongoDB, and Redis.

Key Changes

• Dependency: Added dartssh2: ^4.1.0.
• Core Architecture & Models:
  • SshTunnelConfig and SshTunnelSecrets with support for Password, Private Key (PEM, OpenSSH, RSA, Ed25519) + Passphrase, and SSH Agent authentication ($SSH_AUTH_SOCK / Windows Named Pipe).
  • Multi-hop ProxyJump / Jump Host support.
  • Zero-leak security guarantee: SSH secrets stored strictly in ConnectionSecretsStore (OS Keychain, libsecret, DPAPI), plain SQLite querya.db stores only non-sensitive config in driverOptions['ssh_tunnel']. Immediate in-memory scrubbing/zeroing after authentication.
• Ephemeral Port Forwarding & Connection Pooling:
  • SshTunnelManager: Binds local ephemeral loopback port (127.0.0.1:0), establishes SSH channel forwarding to remote database host/port.
  • Reference-counting session pool for sharing bastions across multiple queries, tabs, and database connections.
  • Keep-alive ping mechanism (ServerAliveInterval) and graceful teardown on close/disconnect.
• Database Adapters:
  • Integrated tunnel forwarding in PostgresConnection, MysqlConnection, MongoConnection, and RedisConnection.
• UI & Forms:
  • Reusable SshTunnelSection widget with interactive authentication selectors, file picker for private keys, fingerprint validation, and a "Test SSH Connection" button.
  • Integrated into PostgreSQL, MySQL, MongoDB, and Redis connection forms.
• Tests:
  • Added unit tests in test/core/security/ssh_tunnel_test.dart covering configuration, secret scrubbing, and ConnectionRow serialization.
  • Updated test/core/storage/local_db_secrets_test.dart verifying secure store persistence and secrets merging.

Closes #1031