feat: context menu improvements for header, FK cells, and empty space

[datlechin]

Summary

Three context menu improvements identified from a full audit:

1. Column header menu — Sort + Show All Columns

• Added Sort Ascending and Sort Descending at the top of the header right-click menu
• Added Show All Columns (only visible when columns are hidden) after Hide Column
• Uses existing onSort callback and new onShowAllColumns callback

2. Data grid row menu — FK Preview & Navigate

• When right-clicking a FK column cell with a non-empty value, two new items appear:
  • Preview Referenced Row — opens the FK preview popover (same as Cmd+Enter)
  • Open {table} — navigates directly to the referenced table
• Items only appear for FK columns with non-null values

3. Empty space — Add Row

• Right-clicking empty space below data grid rows shows Add Row (editable tables only)
• Uses the existing emptySpaceMenu infrastructure from the Structure tab PR
• Reuses StructureMenuTarget for action lifecycle management

Test plan

• Right-click column header → Sort Ascending/Descending → verify sorting works
• Hide a column → right-click another header → verify "Show All Columns" appears
• Click "Show All Columns" → verify all hidden columns reappear
• Right-click a FK cell with value → verify "Preview Referenced Row" and "Open {table}"
• Click "Preview Referenced Row" → verify popover appears
• Click "Open {table}" → verify navigation to referenced table
• Right-click a non-FK cell → verify no FK items in menu
• Right-click a FK cell with NULL value → verify no FK items
• Right-click empty space in editable table → verify "Add Row"
• Right-click empty space in query results → verify no menu