Add connection health monitoring with auto-reconnect

[datlechin]

Summary

• Add periodic health checks (30s SELECT 1 ping) for MySQL/MariaDB and PostgreSQL connections via a new ConnectionHealthMonitor actor
• Auto-reconnect with exponential backoff (3 retries: 2s, 4s, 8s) when a health check fails
• Toolbar Reconnect button (↻) appears when connection is in error or disconnected state for manual recovery
• Store effectiveConnection on ConnectionSession so SSH-tunneled connections auto-reconnect through the tunnel (not directly to remote host)

Architecture

• ConnectionHealthMonitor — new Swift actor with closure-based DI (mirrors SSHTunnelManager pattern), manages ping loop + retry state machine
• DatabaseManager — stores monitors per session, creates/destroys on connect/disconnect, exposes reconnectCurrentSession() for manual reconnect
• Toolbar — new .reconnect item with arrow.triangle.2.circlepath SF Symbol, enabled only in error/disconnected states

Files Changed

File	Change
Core/Database/ConnectionHealthMonitor.swift	NEW — Actor with ping loop, retry logic, state machine (229 lines)
Core/Database/DatabaseManager.swift	Health monitor lifecycle, reconnectDriver() helper, reconnectCurrentSession()
Models/ConnectionSession.swift	Added effectiveConnection for SSH tunnel awareness
Views/Toolbar/ToolbarItemIdentifier.swift	Added .reconnect case
Views/Toolbar/ToolbarItemFactory.swift	Added makeReconnectItem() + action proxy
Views/Toolbar/ToolbarController.swift	Added reconnect to defaults + validation
Views/Main/MainContentNotificationHandler.swift	Added .reconnectDatabase observer
OpenTableApp.swift	Added .reconnectDatabase notification name

Test plan

• Connect to MySQL via direct connection, kill mysqld, observe auto-reconnect in toolbar status
• Connect via SSH tunnel, kill database process (not tunnel), verify auto-reconnect uses tunnel
• After 3 failed retries, verify Reconnect button appears and is clickable
• Click Reconnect button, verify manual reconnection works and schema reloads
• Verify SQLite connections don't have health monitoring (no unnecessary pings)
• Disconnect manually — verify health monitor stops cleanly