refactor(nav): replace static back-link hrefs with BackButton

All 7 in-scope back navigation links converted to use history.back().
Admin panel mobile chevron converted inline (icon-only, different
visual pattern). Cancel buttons left as static <a> links.

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

frontend/src/routes/documents/[id]/edit/+page.svelte

@@ -2,6 +2,7 @@
 import { enhance } from '$app/forms';
 import { m } from '$lib/paraglide/messages.js';
 import { getConfirmService } from '$lib/services/confirm.svelte.js';
+import BackButton from '$lib/components/BackButton.svelte';
 import DocumentEditLayout from '$lib/components/document/DocumentEditLayout.svelte';
 
 let { data, form } = $props();
@@ -26,18 +27,7 @@ async function handleDelete() {
 
 <DocumentEditLayout doc={doc} formId="update-form" formAction="?/update" formError={form?.error}>
 	{#snippet topbar()}
-		<a
-			href="/documents/{doc.id}"
-			class="group inline-flex items-center font-sans text-xs font-bold tracking-widest text-ink-2 uppercase transition-colors hover:text-ink"
-		>
-			<img
-				src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Arrow/Arrow-Left-MD.svg"
-				alt=""
-				aria-hidden="true"
-				class="mr-2 h-4 w-4 transform transition-transform group-hover:-translate-x-1"
-			/>
-			{m.btn_back_to_document()}
-		</a>
+		<BackButton />
 
 		<p class="max-w-sm truncate text-center font-serif text-sm font-medium text-ink">
 			{doc.title || doc.originalFilename}