Implement SchemaActions and action handlers in the webview

[neumaennl]

Implement SchemaActions and their handlers in the webview.

• Define SchemaActions for supported editing commands (add, remove, edit for elements, attributes, etc.)
• Implement handler functions to translate UI actions into commands dispatched to the VS Code extension
• Ensure robust type protection for actions and handlers
• Wire actions through the webview’s event system

Acceptance criteria:

• Each supported user editing operation in the UI maps to a SchemaAction
• Handlers are covered by unit tests and/or integration tests
• Scheme supports future extension for additional UI operations
• All command dispatches are type-safe
• Linked to the UI/UX concept and design (Create UI/UX concept and mock-ups for Phase 3 editor integration #153)