v0.4.0

Highlights:
- Multi-engine support: PostgreSQL and MySQL alongside SQL Server
- Microsoft Entra ID auth for Azure SQL Database
- SSH tunneling with auto-reconnect on idle
- Visual execution plan tree across all engines
- Per-tab connection context
- AI assistant sees active query editor contents
- Resizable/collapsible sidebar
- Results grid: copy-as-JSON, hardened SAVE_TO_FILE, column filtering fixes
- SQL dialect conversion via @memberjunction/sqlglot-ts
- Flyway/Skyway placeholder detection
- Migrated test suite from Jest to Vitest, GitHub Actions CI