Mirrors the #780 layer-filter e2e for #827: switching Datum/Ereignis/Thema
issues zero extra GET /api/timeline (REQ-002), the control stays overflow-free
and ≥44px with full-word aria-labels at 320px (REQ-011), and a 320px axe pass
holds in light and dark mode (REQ-010g). Local-only like the filter e2e (E2E is
not yet in CI).
Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>