feat: enhance field editor with date/time pickers, FK validation, and UX improvements (#53)

## Summary

This PR significantly improves the field editing experience in the side
panel with new date/time pickers, foreign key field editing and
validation, and better NULL handling. All nullable fields are now
immediately editable without needing a NULL toggle first.

## Key Changes

- **New Date/Time Pickers**: DateTimePicker, DatePicker, and TimePicker
components with scroll-wheel time control, year/month navigation, and
quick action buttons (Today, Now, UTC)
- **FK Field Editing**: Enable editing of foreign key fields with inline
validation button that checks if the value exists in the referenced
table
- **Improved NULL Button**: Show NULL button for all nullable fields
with cleaner icon-only design; only disable input when field is both
nullable AND null
- **INSERT SQL Preview**: Display formatted INSERT statements in pending
changes panel with syntax highlighting
- **Better Defaults**: Numeric/date/time fields default to empty string
(not null) for immediate editability during inserts
- **Visual Enhancements**: Field type icons, FK badges with references,
required field indicators

## Test Plan

- [x] Create new rows with datetime fields - should be immediately
editable
- [x] Create new rows with FK fields - should show FK badge and allow
editing
- [x] Click FK check button - should validate value exists in referenced
table
- [x] Click NULL button on nullable fields - should toggle between value
and NULL
- [x] View pending INSERT/UPDATE/DELETE changes - should show formatted
SQL

Author: danielss-dev