refactor: native UndoManager responder chain integration

[datlechin]

Summary

Complete rewrite of the undo/redo architecture from a hacky 6-layer manual routing system to native AppKit responder chain integration.

• Each QueryTab owns its own DataChangeManager (with its own UndoManager), preserving undo history across tab switches
• Undo closures capture RowBuffer directly, mutating both change tracking state and resultRows in one step (eliminates UndoResult side-channel)
• KeyHandlingTableView exposes undoManager through the responder chain — AppKit routes Cmd+Z natively
• Edit menu shows dynamic action names ("Undo Edit Cell", "Undo Delete Row")
• Eliminated: UndoResult, UndoAction, applyDataUndo monolith, TabPendingChanges, 6-layer routing chain, manual first-responder fork in menu

Before:

Cmd+Z → Menu fork (firstResponder check)
      → MainContentCommandActions.undoChange()
      → MainContentCoordinator.undoLastChange()
      → RowOperationsManager.undoLastChange()
      → DataChangeManager.undoLastChange() → private undoManager.undo()
      → applyDataUndo() sets lastUndoResult
      → UndoResult returned → applyUndoResult() mutates resultRows

After:

Cmd+Z → AppKit responder chain
      → KeyHandlingTableView.undoManager (override)
      → DataChangeManager.undoManager
      → registered closure fires (updates state + mutates rows + triggers UI)

42 files changed, -253 lines net

Test plan

• Edit cell → Cmd+Z undoes → Cmd+Shift+Z redoes
• Insert row → Cmd+Z removes → Cmd+Shift+Z re-inserts
• Delete row → Cmd+Z undeletes
• Switch tabs with unsaved edits, switch back → Cmd+Z still works
• Edit menu shows "Undo Edit Cell" / "Undo Delete Row" dynamically
• Edit menu Undo greys out when nothing to undo
• SQL editor: Cmd+Z undoes text edits (unaffected)
• Structure tab: Cmd+Z undoes structure changes
• Quit with unsaved changes: still detects correctly
• Batch delete rows → Cmd+Z restores all