feat: drag to reorder columns in Structure tab

[datlechin]

Summary

• Add drag-and-drop column reordering in the Structure tab (Columns sub-tab)
• MySQL/MariaDB only — uses ALTER TABLE ... MODIFY COLUMN ... AFTER
• Executes immediately on drop, reloads from DB on success
• Disabled when there are unsaved structure changes

Closes #489
Ref: #488

Test plan

• Open a MySQL/MariaDB table structure, drag a column up/down
• Verify ALTER TABLE executes and column appears at new position
• Verify drag is disabled for PostgreSQL/SQLite connections
• Verify drag is disabled when there are pending structure changes
• Verify error alert on SQL failure