feat: add context menu for Structure tab

[datlechin]

Summary

• Add dedicated context menu for the Structure tab, replacing the inherited Results-tab menu that showed irrelevant actions (Copy as INSERT, Export Results, Set Value)
• New StructureRowViewWithMenu (NSTableRowView subclass) with schema-appropriate actions
• Add rowViewProvider to DataGridView so callers can inject custom row views
• Add generateColumnDefinitionSQL, generateIndexDefinitionSQL, generateForeignKeyDefinitionSQL to plugin protocol for SQL definition generation
• Implement in MySQL and PostgreSQL plugins (other databases gracefully degrade)

Context menu per tab

Columns / Indexes:

• Copy Name
• Copy Definition (database-specific SQL syntax)
• Duplicate / Delete

Foreign Keys:

• Copy Name
• Copy Definition (SQL syntax)
• Open referenced table (navigates to FK target)
• Duplicate / Delete

DDL / Parts: No context menu

Test plan

• MySQL/PostgreSQL: Structure tab → Columns → right-click → verify menu shows Copy Name, Copy Definition, Duplicate, Delete
• Copy Name → paste → verify column name copied
• Copy Definition → paste → verify SQL syntax (e.g., name VARCHAR(255) NOT NULL DEFAULT '')
• Indexes tab → right-click → Copy Definition → verify index SQL
• Foreign Keys tab → right-click → "Open {table}" → navigates to referenced table
• DDL tab → right-click → no context menu appears
• Read-only view → right-click → Duplicate/Delete not shown, Copy Name/Definition still work
• Deleted row → right-click → only "Undo Delete" shown
• Multi-select rows → right-click → actions apply to all selected