feat: add Server Dashboard & Activity Monitor

[datlechin]

Summary

• New Server Dashboard tab type showing live server state with auto-refresh
• Active Sessions panel: PID, user, database, state, duration, running query with Kill/Cancel actions (confirmation alert)
• Server Metrics panel: Connection count, cache hit ratio, database size, uptime, active queries — displayed as horizontal metric cards with SF Symbol icons
• Slow Queries panel: Collapsible list of queries running > 1 second with duration highlighting
• Auto-refresh: Configurable interval (1s/2s/5s/10s/30s/off), default 5s, pause/resume
• 6 database providers with graceful degradation:
  • PostgreSQL: full support (sessions, metrics, slow queries, kill, cancel)
  • MySQL/MariaDB: full support
  • MSSQL: sessions + metrics + slow queries + kill
  • ClickHouse: sessions + metrics + slow queries + kill
  • DuckDB: metrics only
  • SQLite: metrics only
• Entry points: menu bar ("Server Dashboard") + toolbar button
• One dashboard tab per window (deduplicates on open)
• Tab persistence across sessions

Architecture

• ServerDashboardQueryProvider protocol — each database owns both query SQL and result parsing
• ServerDashboardViewModel (@mainactor @observable) — Task-based refresh loop, per-panel error handling
• Follows ER Diagram tab pattern exactly (TabType, QueryTabManager, coordinator extension, per-tab VM dict)
• 16 new files, 8 modified files, 1,628 lines added

Test plan

• PostgreSQL: verify sessions, metrics, slow queries panels populate
• MySQL: verify sessions, metrics, slow queries panels populate
• SQLite: verify only metrics panel visible, others hidden
• Auto-refresh: change interval, pause/resume, manual refresh (Cmd+R)
• Kill session: confirmation alert → execute → list refreshes
• Tab persistence: close and reopen, dashboard tab restores
• Menu "Server Dashboard" disabled for NoSQL databases (Redis, MongoDB)
• Toolbar dashboard button disabled when disconnected