feat(conversations): add summary with document count and year range (#31)

Shows a summary line above the conversation listing with total document
count and the year span, e.g. "4 Dokumente · 1923–1965".

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

frontend/messages/de.json

@@ -141,6 +141,7 @@
 	"conv_no_results_heading": "Keine Dokumente gefunden.",
 	"conv_no_results_text": "Versuchen Sie, den Zeitraum anzupassen.",
 	"conv_swap_btn": "Personen tauschen",
+	"conv_summary": "{count} Dokumente · {yearFrom}–{yearTo}",
 
 	"admin_heading": "Admin Dashboard",
 	"admin_tab_users": "Benutzer",

frontend/messages/en.json

@@ -141,6 +141,7 @@
 	"conv_no_results_heading": "No documents found.",
 	"conv_no_results_text": "Try adjusting the time period.",
 	"conv_swap_btn": "Swap persons",
+	"conv_summary": "{count} documents · {yearFrom}–{yearTo}",
 
 	"admin_heading": "Admin Dashboard",
 	"admin_tab_users": "Users",

frontend/messages/es.json

@@ -141,6 +141,7 @@
 	"conv_no_results_heading": "No se encontraron documentos.",
 	"conv_no_results_text": "Intente ajustar el período de tiempo.",
 	"conv_swap_btn": "Intercambiar personas",
+	"conv_summary": "{count} documentos · {yearFrom}–{yearTo}",
 
 	"admin_heading": "Panel de administración",
 	"admin_tab_users": "Usuarios",

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

@@ -14,6 +14,14 @@ let fromDate = $state(untrack(() => data.filters.from));
 let toDate = $state(untrack(() => data.filters.to));
 let sortDir = $state(untrack(() => data.filters.dir));
 
+const documentYears = $derived(
+	data.documents
+		.map((doc) => (doc.documentDate ? new Date(doc.documentDate).getFullYear() : null))
+		.filter((y): y is number => y !== null)
+);
+const yearFrom = $derived(documentYears.length > 0 ? Math.min(...documentYears) : null);
+const yearTo = $derived(documentYears.length > 0 ? Math.max(...documentYears) : null);
+
 // Sync with server data after navigation
 $effect(() => {
 	senderId = data.filters.senderId;
@@ -191,6 +199,19 @@ function swapPersons() {
 			<p class="mt-2 text-sm text-gray-400">{m.conv_no_results_text()}</p>
 		</div>
 	{:else}
+		<!-- Summary bar -->
+		<div class="mb-4">
+			{#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 })}
+				</p>
+			{:else}
+				<p data-testid="conv-summary" class="font-sans text-sm font-medium text-brand-navy/70">
+					{data.documents.length}
+				</p>
+			{/if}
+		</div>
+
 		<!-- CHAT CONTAINER -->
 		<div class="relative overflow-hidden rounded-sm border border-brand-sand bg-white shadow-sm">
 			<!-- Decoration: Central Timeline Line -->