fix: resolve high CPU usage and memory leaks at idle (#368)

[datlechin]

Summary

Root cause: A cascading UI re-render loop where every 30-second health monitor ping, every tab switch, and every window focus change triggered unconditional activeSessions dictionary writes that invalidated all open windows' views — even when nothing meaningful changed.

Changes (16 fixes across 15 files)

Critical — Idle CPU elimination:

• Guard updateSession() against no-op writes using isContentViewEquivalent + driver identity comparison
• Skip onStateChanged callback for routine healthy↔checking health monitor ping cycles
• Route switchToSession through guarded updateSession (lastActiveAt-only changes are no-ops)
• Add per-connection connectionStatusVersions dictionary so each window observes only its own connection's counter

High — Correctness & Performance:

• Track all driver operations (fetchTables, fetchColumns, executeSchemaChanges) in queriesInFlight to prevent health monitor TOCTOU races on shared non-thread-safe C driver handles
• Fix activeCoordinators memory leak by moving registerForPersistence() from init() to markActivated()
• Move DatabaseRowProvider.prefetchRows DB fetch off MainActor with tracked task and dedup guard

Medium — Startup & UI:

• Move Keychain migration to Task.detached for non-blocking startup
• Move plugin bundle.load() off MainActor via nonisolated static method
• Replace loadConnections() disk I/O in ContentView.init with in-memory lookup
• Change WindowLifecycleMonitor to weak NSWindow references with purgeStaleEntries()
• Guard redundant .connecting status writes during reconnect attempts
• Use targeted reloadData(forRowIndexes:columnIndexes:) for FK metadata instead of full table reload
• Debounce JSON syntax highlighting with 100ms DispatchWorkItem
• Replace O(n²) undo batch index shift with O(n log n) binary search
• Add [weak self] to nested DispatchQueue.main.async in SQLEditorCoordinator

Performance impact

With 5 open connections at idle:

• Before: 10 MainActor dispatches + 10 onChange closures evaluated every 30 seconds, plus full re-render storm on every tab switch
• After: Zero MainActor dispatches during healthy ping cycles; updateSession no-ops skip dictionary writes entirely; each window only observes its own connection

Test plan

• Open 3+ database connections in separate windows
• Leave app idle for 2+ minutes — verify CPU stays near 0% in Activity Monitor
• Switch between connection windows — verify no visual flicker in other windows
• Disconnect one connection — verify window closes, others unaffected
• Open ConnectionSwitcherPopover — verify all sessions listed correctly
• Reconnect a connection — verify lazy-load fires correctly
• Scroll large result sets — verify smooth prefetch (no main thread blocking)
• Edit JSON cell — verify syntax highlighting responds smoothly
• Undo batch row insertion (100+ rows) — verify no UI freeze

Summary by CodeRabbit

• Bug Fixes

  • Better window lifecycle handling to avoid memory leaks and stale window entries
  • Reduced unnecessary UI updates for connection health/state transitions

• Performance

  • Background plugin loading and improved startup responsiveness
  • Debounced JSON syntax highlighting for smoother typing
  • Targeted data-grid refreshes for foreign-key columns only
  • Cancellable, windowed prefetching for faster, more stable data loading