feat: add FK preview popover on Cmd+Enter in data grid

[datlechin]

Summary

• Press Cmd+Enter on a foreign key cell in the data grid to preview the referenced row in a popover
• Shows all columns of the referenced row as key-value pairs with loading/error/null/not-found states
• Includes "Open {table}" button to navigate to the referenced table with filter applied
• Reuses existing FK infrastructure — no protocol or model changes needed

Closes #556

Test plan

• Connect to MySQL/PostgreSQL with FK tables
• Open a table with FK columns → focus an FK cell → press Cmd+Enter → popover shows referenced row
• Test with NULL FK value → shows "NULL — no referenced row"
• Test with dangling FK (value not in referenced table) → shows "Referenced row not found"
• Click "Open {table}" button → navigates to referenced table with filter
• Press Cmd+Enter on a non-FK column → nothing happens
• Verify Cmd+Enter still executes query in the SQL editor (different responder)