fix(forms): correct required/optional field markers and divider placement

- Add * to Datum and Absender labels (both are required fields)
- Add required prop to PersonTypeahead to show * in its label
- Move "Optional" divider in DescriptionSection to after Titel (the only
  required field), so Tags and Inhalt appear below the divider where they belong

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

frontend/src/lib/components/document/WhoWhenSection.svelte

@@ -59,7 +59,7 @@ $effect(() => {
 		<!-- Datum (required — row 1, col 1) -->
 		<div>
 			<label for="documentDate" class="mb-1 block text-sm font-medium text-ink-2"
-				>{m.form_label_date()}</label
+				>{m.form_label_date()}*</label
 			>
 			<input
 				id="documentDate"
@@ -85,6 +85,7 @@ $effect(() => {
 			<PersonTypeahead
 				name="senderId"
 				label={m.form_label_sender()}
+				required={true}
 				bind:value={senderId}
 				initialName={initialSenderName}
 				suggestedName={suggestedSenderName}