Parses ?undated strictly (=== 'true', mirroring the tagOp clamp), forwards
it as undated || undefined so the absent case drops out of the query, and
returns the flag in page data for the control to reflect. Adds the
docs_filter_undated_only toggle label and the explanatory
docs_range_excludes_undated empty-state copy in de/en/es. The badge reuses
the existing date_precision_unknown ("Datum unbekannt") key from #677.
OpenAPI types hand-edited for the new undated query param on /search and
/ids — CI must run `npm run generate:api` to confirm parity with the spec.
Refs #668
f1fc3dc1ce