Design and implement toolbar and context menus for editing

[neumaennl]

Design and implement toolbar and context menus for editing in the schema diagram webview.

• Define user workflows and common actions for toolbar and context menus based on UI/UX concept (Create UI/UX concept and mock-ups for Phase 3 editor integration #153)
• Add buttons and options for adding, removing, and editing schema components
• Implement menus/context menus accessible via right click or keyboard
• Ensure all actions are integrated with selection logic and SchemaActions

Acceptance criteria:

• Toolbar and context menus exist with actions mapped to editing operations
• Accessibility and keyboard navigation are tested
• Menus/context menus extensible for future features
• Linked to UI/UX concept issue (Create UI/UX concept and mock-ups for Phase 3 editor integration #153)