feat(behandelplan): E3 UI componenten en documentatie updates

Behandelplan UI (E3):
- page-client.tsx: Client-side behandelplan pagina
- actions.ts: Server actions voor CRUD operaties
- behandelplan-view.tsx: Volledige behandelplan weergave
- behandelplan-list.tsx: Lijst van behandelplannen
- editable-section.tsx: Herbruikbare edit sectie component
- sections/: Goal, intervention en behandelstructuur forms

UI Componenten:
- components/ui/checkbox.tsx (shadcn)
- components/ui/input.tsx (shadcn)
- components/ui/select.tsx (shadcn)

Documentatie:
- agenda-systeem.mdx toegevoegd
- _index.json en metadata.json bijgewerkt

Dependencies:
- @radix-ui/react-checkbox toegevoegd

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -20,11 +20,13 @@
     "@fullcalendar/timegrid": "^6.1.19",
     "@hookform/resolvers": "^5.2.2",
     "@radix-ui/react-alert-dialog": "^1.1.15",
+    "@radix-ui/react-checkbox": "^1.3.3",
     "@radix-ui/react-dialog": "^1.1.15",
     "@radix-ui/react-dropdown-menu": "^2.1.16",
     "@radix-ui/react-icons": "^1.3.2",
     "@radix-ui/react-label": "^2.1.8",
     "@radix-ui/react-progress": "^1.1.8",
+    "@radix-ui/react-select": "^2.2.6",
     "@radix-ui/react-slider": "^1.3.6",
     "@radix-ui/react-slot": "^1.2.4",
     "@radix-ui/react-tabs": "^1.1.13",