refactor(datagrid): Phase 3 keyboard interaction cleanup

[datlechin]

Summary

DataGrid Phase 3 — full keyboard interaction refactor. Removes shadow handlers and custom reimplementations of NSTableView built-ins.

Key changes:

• Remove Ctrl+HJKL Vim navigation (shadowed system text editing actions deleteBackward/insertNewlineIgnoringFieldEditor/deleteToEndOfLine/centerSelectionInVisibleArea)
• Remove custom Shift+Up/Down arrow handlers (NSTableView native handles range selection via moveDownAndModifySelection: et al)
• Remove custom Home/End/PageUp/PageDown handlers (NSTableView native via interpretKeyEvents)
• Remove orphaned selectionAnchor/selectionPivot state (NSTableView manages internal anchor)
• Add focusedRow sync after native row navigation in tableViewSelectionDidChange
• Remove redundant DataChangeManager.undoLastChange/redoLastChange public API (production uses window.undoManager via responder chain)
• Update audit doc with Phase 3 status + per-issue annotations

Net delta: -250 lines

Why

Audit re-evaluated. Most Phase 3 audit issues found to be already correct in current code (Phase 1/2 era fixes). Real Phase 3 changes:

• feat: Native macOS inline error banners (replace blocking dialogs) #12 Ctrl+HJKL shadows system shortcuts
• Depfu Error: Depfu is stuck and needs your help #34 Custom Home/End/PageUp/PageDown reimplemented native NSTableView behavior
• Add language setting with full Vietnamese localization #49 Custom Shift+Arrow reimplemented NSTableView native
• Structure View Refactor & Keyboard Handling Migration #24 NSUndoManager dual-invocation (cleanup of dead public API)

Already correct (no changes): #6, #10, #11, #13, #14, #16, #23, #32, #33, #35.

See docs/development/datagrid-audit.md Phase 3 Status section.

Behaviors preserved

• Tab/Shift+Tab cell navigation (NSTableView has no native cell-by-cell)
• Left/Right arrow column focus (NSTableView is row-oriented)
• Return/Enter triggers inline edit
• FK preview keyboard shortcut (configurable)
• Escape cancels active edit (does NOT deselect rows)
• focusedRow/focusedColumn separate from row selection (TablePlus pattern)

Test plan

• xcodebuild test (user runs locally; SIGTERM in CI is pre-existing test infra issue unrelated to this PR)
• swiftlint lint --strict
• Manual: Up/Down arrows move selection by 1 row
• Manual: Shift+Up/Down extends selection
• Manual: Home jumps to row 0, End jumps to last row
• Manual: Page Up/Down scrolls one page
• Manual: Tab/Shift+Tab navigate cells
• Manual: Cmd+Z/Cmd+Shift+Z undo/redo via menu bar
• Manual: Multi-cell paste (5x3 grid into smaller-than-table region)
• Manual: Ctrl+H during cell edit deletes character backward (not column nav)
• Manual: Right-click on active edit shows row context menu