feat(ui): hide write UI from users without WRITE_ALL permission

Wrap write-only elements with {#if data.canWrite} in:
- Home page: Neues Dokument link
- Persons list: Neue Person link
- Document detail: Bearbeiten button
- Person detail: edit button, edit form, merge section

Refs #17
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/page.svelte.spec.ts

@@ -17,6 +17,7 @@ vi.stubGlobal(
 
 const emptyData = {
 	user: undefined,
+	canWrite: true,
 	filters: { q: '', from: '', to: '', senderId: '', receiverId: '', tags: [] },
 	documents: [],
 	initialValues: { senderName: '', receiverName: '' },