feat(documents): show global undated count chip on the filter toggle

Surface the backend's global undatedCount on the "Nur undatierte" toggle as
a count chip — the total undated documents matching the current filter
across all pages, not the page slice. The loader forwards undatedCount
straight through (defaulting to 0); the chip hides at 0 and stays visible
regardless of the toggle state so it advertises the triage backlog size.

generate:api was hand-edited (undatedCount added to DocumentSearchResult) —
CI must re-run npm run generate:api to confirm parity.

Refs #668

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/routes/documents/+page.server.ts

@@ -85,6 +85,7 @@ export async function load({ url, fetch }) {
 			pageNumber: 0,
 			pageSize: PAGE_SIZE,
 			totalPages: 0,
+			undatedCount: 0,
 			q,
 			from,
 			to,
@@ -116,6 +117,8 @@ export async function load({ url, fetch }) {
 		pageNumber: result.data?.pageNumber ?? page,
 		pageSize: result.data?.pageSize ?? PAGE_SIZE,
 		totalPages: result.data?.totalPages ?? 0,
+		// Global undated count for the active filter, across all pages (issue #668).
+		undatedCount: result.data?.undatedCount ?? 0,
 		q,
 		from,
 		to,