feat(documents): explain that a date range excludes undated documents

DocumentList gains from/to props; when a date range is active and yields no
results, the empty state shows the localized docs_range_excludes_undated
note instead of the generic copy, so the reader understands undated letters
aren't part of a range. Person-grouped modes keep undated letters under
their sender/receiver (badge-on-row, no synthetic sub-group).

Refs #668

frontend/src/routes/DocumentList.svelte

@@ -15,7 +15,9 @@ let {
 	error,
 	total = 0,
 	q = '',
-	sort = 'DATE'
+	sort = 'DATE',
+	from = '',
+	to = ''
 }: {
 	items: DocumentListItem[];
 	canWrite: boolean;
@@ -23,8 +25,15 @@ let {
 	total?: number;
 	q?: string;
 	sort?: SortMode;
+	from?: string;
+	to?: string;
 } = $props();
 
+// A from/to range excludes undated documents — when it yields nothing, the
+// empty state must say so explicitly (a localized constant, never a reflected
+// backend string). Issue #668.
+const hasDateRange = $derived(!!from || !!to);
+
 const groups = $derived.by(() => {
 	if (sort === 'SENDER') return groupBySender(items);
 	if (sort === 'RECEIVER') return groupByReceiver(items);
@@ -119,7 +128,13 @@ function groupByReceiver(docItems: DocumentListItem[]) {
 			</div>
 			<h3 class="font-serif text-lg font-medium text-ink">{m.docs_empty_heading()}</h3>
 			<p class="mt-1 font-sans text-sm text-ink-2">
-				{q ? m.docs_empty_for_term({ term: q }) : m.docs_empty_text()}
+				{#if hasDateRange}
+					{m.docs_range_excludes_undated()}
+				{:else if q}
+					{m.docs_empty_for_term({ term: q })}
+				{:else}
+					{m.docs_empty_text()}
+				{/if}
 			</p>
 			<button
 				onclick={() => goto('/documents')}