Editor: Phase 3 - UI Integration

[neumaennl]

Summary:
Enable diagram interactivity for editing (toolbar actions, property panel, webview action creators, selection model). (ADR 001)

Related ADR:
docs/architecture/001-editor-transition.md — Phase 3

Acceptance criteria:

• Webview sends correctly typed commands for edits
• Toolbar actions, context menus, and selection are interactive
• Property panel supports live editing
• Bi-directional feedback for schema errors

Tasks:

• Implement SchemaActions and action handlers
• Toolbars and context menus for add/remove/edit
• Hook property panel to editing commands
• Selection and multi-select behavior