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/documents/+page.svelte

@@ -343,6 +343,8 @@ $effect(() => {
 		canWrite={data.canWrite}
 		error={data.error}
 		sort={sort}
+		from={data.from}
+		to={data.to}
 	/>
 
 	<Pagination page={data.pageNumber} totalPages={data.totalPages} makeHref={buildPageHref} />