fix(ui): unify Briefwechsel search bar with document search card style

Wrap person bar + filter controls in a card matching the document
search page (rounded-sm border p-6 shadow-sm). Switch PersonTypeahead
to default mode with matching label/input overrides. Bump date inputs
and sort button to text-sm py-2.5. Filter row uses border-t separator
like the document search advanced section.

Refs: #179

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

frontend/src/routes/briefwechsel/SinglePersonHintBar.svelte

@@ -19,7 +19,7 @@ let toYear = $derived(toDate ? toDate.substring(0, 4) : '');
 </script>
 
 <div
-	class="flex items-center gap-[5px] border-b border-accent bg-accent-bg px-3 py-[6px] text-xs text-ink"
+	class="mt-4 flex items-center gap-1.5 rounded-sm border border-accent bg-accent-bg px-4 py-2.5 text-sm text-ink"
 >
 	<svg
 		xmlns="http://www.w3.org/2000/svg"