feat: FK navigation for iOS data browser

[datlechin]

Summary

• Tap FK value in row detail to preview referenced row in a sheet
• Long-press row in data browser to see FK navigation options in context menu
• FK metadata fetched once per table alongside column details
• Preview shows full referenced row with LabeledContent layout
• Handles missing referenced rows with empty state

Test plan

• Open table with FK columns (e.g. orders with user_id FK)
• Tap row, see arrow icon next to FK column value
• Tap arrow — sheet shows referenced row from related table
• Long-press row in data browser — context menu shows FK options
• FK to non-existent row — shows "No Referenced Row"
• Table without FKs — no FK UI shown