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.svelte

@@ -268,6 +268,7 @@ $effect(() => {
 		bind:tagQ={tagQ}
 		bind:tagOperator={tagOperator}
 		bind:undated={undated}
+		undatedCount={data.undatedCount ?? 0}
 		initialSenderName={initialSenderName}
 		initialReceiverName={initialReceiverName}
 		navKey={navKey}