Summary

Fixes the multi-column sort part of #541.

Root cause: NSTableView swallows Shift+Click on column headers — the sortDescriptorsDidChange delegate is never called when Shift is held. The existing multi-sort logic in handleSort was correct but unreachable.

Fix: Added SortableTableHeaderView (NSTableHeaderView subclass) that intercepts mouseDown when Shift is held, identifies the clicked column, and routes directly to the coordinator's multi-sort handler — bypassing NSTableView's broken Shift+Click behavior.

Changes:

• KeyHandlingTableView.swift — Added SortableTableHeaderView class
• DataGridView.swift — Installed custom header view on table creation
• DataGridView+Sort.swift — Added handleShiftClickSort entry point for the header view

Test plan

• Click a column header → sorts by that column (single sort, unchanged behavior)
• Shift+Click another column header → adds it as secondary sort (e.g., ORDER BY name ASC, id ASC)
• Shift+Click the same column again → removes it from multi-sort
• Verify sort indicators show priority numbers (e.g., name 1▲, id 2▼)
• Normal click after multi-sort → resets to single-column sort