refactor(datagrid): column index helpers, metadata dedup, keyboard nav, overlay panel

[datlechin]

Summary

• Column index abstraction: replace raw "col_\(index)" string encoding and Int(dropFirst(4)) decoding with columnIdentifier(for:), tableColumnIndex(for:), dataColumnIndex(for:) helpers on DataGridView, rename columnIndex(from:) to dataColumnIndex(from:) for consistency (20+ sites)
• ResultSet metadata dedup: replace 5 stored column metadata properties on ResultSet with computed properties proxying to the shared RowBuffer, remove redundant assignments in QueryHelpers
• Delete DatabaseRowProvider: remove 137 lines of dead code (never instantiated)
• VoiceOver: remove 5,000-row gate on accessibility labels so all datasets get screen reader announcements
• Escape key: cancelOperation clears cell focus only, no longer deselects rows
• Home/End/PageUp/PageDown: add 4 navigation key codes and handlers with Shift+selection support
• Context menu key equivalents: remove Cmd+C/V/D and Backspace shortcuts from context menu (belong on menu bar only)
• CellOverlayEditor: replace NSEvent.addLocalMonitorForEvents click-outside monitor with borderless CellOverlayPanel (NSPanel subclass), click-outside handled natively via resignKey()

Test plan

• Open a table, verify column headers, sorting, and cell editing still work
• Click a cell to focus, press Escape: focus clears but row selection stays
• Press Home/End/PageUp/PageDown to navigate rows, with Shift held for range selection
• Right-click a row: context menu shows no keyboard shortcuts
• Double-click a multiline cell: overlay editor appears as a floating panel, click outside commits
• Open a table with >5,000 rows, enable VoiceOver: cell labels are announced
• Verify multi-statement query results display correctly (ResultSet metadata dedup)

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[chatgpt-codex-connector]

Compute Shift+Page navigation from pivot, not selectedRow

When range-selecting with Shift held, selectedRow is always the first selected row, so using it as the base row causes repeated Shift+PageDown/Shift+PageUp presses to stop extending the selection after the first jump. In practice, once a range like 10...25 is selected, selectedRow stays at 10, so targetRow is recomputed from 10 every time instead of from the current pivot; this breaks expected keyboard range extension behavior for the new PageUp/PageDown handlers.

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