fix: column visibility UX polish (#778)

[datlechin]

Closes #778

Summary

UX polish for column visibility popover. The current approach (toggle checkboxes) is correct and matches DataGrip/DBeaver/TablePlus. These changes improve discoverability.

Changes

• Header shows "5 of 42" when columns are hidden (was just "Columns")
• Status bar shows "Columns (5/42)" instead of "Columns (37 hidden)"
• Popover wider (260px, was 220px) for long column names
• Popover taller (420px, was 350px) for many columns
• Search field shows for 5+ columns (was 10+)

Test plan

• Build passes
• Open table with many columns, verify count badge in header and status bar
• Toggle columns, verify counts update
• Search field visible for tables with 5+ columns