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/lib/generated/api.ts

@@ -2471,6 +2471,8 @@ export interface components {
             pageSize: number;
             /** Format: int32 */
             totalPages: number;
+            /** Format: int64 */
+            undatedCount: number;
         };
         MatchOffset: {
             /** Format: int32 */