…anager

- Fix primaryKeyChange undo closure capturing stale workingPrimaryKey
- Fix batch insertion undo leaking insertedRowData entries
- Clear undo history in loadSchema to prevent stale actions on table switch
- Make undoManager private to prevent bypass of lastUndoResult contract

…ivity crash

NSHostingView wrapping MainContentView creates a separate SwiftUI evaluation
context. When updateNSView sets rootView, the inner hosting view re-evaluates
MainContentView.body, which re-entrantly accesses @observable properties
(QueryTabManager.tabs) already being read by the outer tree — triggering a
Swift exclusivity violation crash.

The PanelResizeHandle approach keeps MainContentView in the same SwiftUI tree,
avoiding re-entrant access.

With groupsByEvent=false, NSUndoManager requires explicit beginUndoGrouping()
before registerUndo(). Without it, registerUndo throws an exception that AppKit
catches silently, causing all undo registration to be no-ops. This broke cell
edit commit — changes were recorded in the tracking arrays but never registered
with the undo manager.

The default groupsByEvent=true auto-creates groups per run loop pass, which is
correct for individual cell edits.

- rowInsertion: capture savedValues BEFORE undoRowInsertion clears them,
  register reverse AFTER redo repopulates insertedRowData
- batchRowInsertion redo: restore insertedRowData for each re-inserted row
  so SQL generation produces correct INSERT values

StructureChangeManager:
- S1: Distinguish undo-of-new-add vs redo-of-existing-delete in columnAdd/Delete,
  indexAdd/Delete, foreignKeyAdd/Delete by checking currentColumns/Indexes/FKs
- S2: Update pendingChanges for newly-added items on undo of edit (add else branches)
- S3: Call validate() after every undo/redo to keep validationErrors fresh

DataChangeManager:
- D1: Populate insertedRowData in batchRowInsertion redo path
- D2: Shift existing row indices up when redoing row insertion (new shiftRowIndicesUp helper)
- D3: Shift insertedRowData and modifiedCells keys in undoRowInsertion
- D4: Use rows.contains instead of rows.first for batch deletion undo/redo detection
- D5: Set groupsByEvent=false with explicit beginUndoGrouping/endUndoGrouping
  via registerUndoAction helper to prevent run-loop auto-grouping

…registration failures