feat(persons): retarget frequent-correspondents card to document search

The "Häufige Korrespondenten" card linked into the standalone Briefwechsel
view. Retarget each chip to the existing document search pre-filtered by
sender and receiver (/documents?senderId=A&receiverId=B), naming both
persons in a search-action title, swapping the chat-bubble icon for a
magnifier, and clarifying that the ×N badge counts shared letters in both
directions (not the unidirectional search result count).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/persons/[id]/+page.svelte

@@ -68,7 +68,11 @@ const coCorrespondents = $derived.by(() => {
 
 		<!-- Right column: correspondents + relationships + documents -->
 		<div>
-			<CoCorrespondentsList coCorrespondents={coCorrespondents} personId={person.id} />
+			<CoCorrespondentsList
+				coCorrespondents={coCorrespondents}
+				personId={person.id}
+				personName={person.displayName}
+			/>
 
 			<div class="mt-6">
 				<PersonRelationshipsCard