fix(enrich): restore fixed-position layout and done icon

Re-applies the scroll fix from 0d3c557 which was missing from this branch:
- measure header height at mount, use it as top offset instead of hardcoded 68px
- fix done page icon to Check-Double-LG

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

frontend/src/routes/enrich/done/+page.svelte

@@ -7,7 +7,7 @@ import { m } from '$lib/paraglide/messages.js';
 		class="border-brand-sand flex flex-col items-center justify-center rounded-sm border bg-white py-20 text-center shadow-sm"
 	>
 		<img
-			src="/degruyter-icons/Simple/Large-32px/SVG/Action/Check/Check-Circle-LG.svg"
+			src="/degruyter-icons/Simple/Large-32px/SVG/Action/Check/Check-Double-LG.svg"
 			alt=""
 			aria-hidden="true"
 			class="mb-6 h-16 w-16"