feat(conversations): add new document link pre-filled with both persons (#33)

Adds a link next to the summary that navigates to the new-document form
with senderId and receiverId pre-filled from the current conversation.

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

frontend/messages/de.json

@@ -142,6 +142,7 @@
 	"conv_no_results_text": "Versuchen Sie, den Zeitraum anzupassen.",
 	"conv_swap_btn": "Personen tauschen",
 	"conv_summary": "{count} Dokumente · {yearFrom}–{yearTo}",
+	"conv_new_doc_link": "Neues Dokument in dieser Korrespondenz",
 
 	"admin_heading": "Admin Dashboard",
 	"admin_tab_users": "Benutzer",

frontend/messages/en.json

@@ -142,6 +142,7 @@
 	"conv_no_results_text": "Try adjusting the time period.",
 	"conv_swap_btn": "Swap persons",
 	"conv_summary": "{count} documents · {yearFrom}–{yearTo}",
+	"conv_new_doc_link": "New document in this correspondence",
 
 	"admin_heading": "Admin Dashboard",
 	"admin_tab_users": "Users",

frontend/messages/es.json

@@ -142,6 +142,7 @@
 	"conv_no_results_text": "Intente ajustar el período de tiempo.",
 	"conv_swap_btn": "Intercambiar personas",
 	"conv_summary": "{count} documentos · {yearFrom}–{yearTo}",
+	"conv_new_doc_link": "Nuevo documento en esta correspondencia",
 
 	"admin_heading": "Panel de administración",
 	"admin_tab_users": "Usuarios",

frontend/src/routes/conversations/+page.svelte

@@ -211,7 +211,7 @@ const enrichedDocuments = $derived(
 		</div>
 	{:else}
 		<!-- Summary bar -->
-		<div class="mb-4">
+		<div class="mb-4 flex items-center justify-between">
 			{#if yearFrom !== null && yearTo !== null}
 				<p data-testid="conv-summary" class="font-sans text-sm font-medium text-brand-navy/70">
 					{m.conv_summary({ count: data.documents.length, yearFrom, yearTo })}
@@ -221,6 +221,17 @@ const enrichedDocuments = $derived(
 					{data.documents.length}
 				</p>
 			{/if}
+			<a
+				data-testid="conv-new-doc-link"
+				href="/documents/new?senderId={senderId}&receiverId={receiverId}"
+				class="inline-flex items-center gap-1 text-sm font-medium text-brand-navy/60 transition-colors hover:text-brand-navy"
+			>
+				<svg class="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+					<path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4"
+					></path>
+				</svg>
+				{m.conv_new_doc_link()}
+			</a>
 		</div>
 
 		<!-- CHAT CONTAINER -->