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

@@ -191,6 +191,7 @@ $effect(() => {
 
 	<!-- DOCUMENT LIST HEADER -->
 	<div class="mb-2 flex justify-end">
+		{#if data.canWrite}
 		<a
 			href="/documents/new"
 			class="inline-flex items-center gap-1 text-sm font-medium text-brand-navy/60 transition-colors hover:text-brand-navy"
@@ -198,6 +199,7 @@ $effect(() => {
 			<img src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Add/Add-General-MD.svg" alt="" aria-hidden="true" class="h-4 w-4" />
 			{m.docs_btn_new()}
 		</a>
+		{/if}
 	</div>
 
 	<!-- DOCUMENT LIST -->