Hook up property panel to schema editing commands

[neumaennl]

Connect the property panel UI with editing commands in the webview and extension.

• Implement two-way binding between property panel fields and schema data
• Validate property panel edits and translate to SchemaActions/commands
• Provide instant feedback for invalid or incomplete data
• Ensure changes in the document are reflected in the property panel UI

Acceptance criteria:

• Property panel supports live editing and reflects VS Code schema changes
• Validation errors are user-friendly and actionable
• Code is organized for testability and extensibility
• Linked to the UI/UX concept issue (Create UI/UX concept and mock-ups for Phase 3 editor integration #153)