Lift smartMode to documents/+page.svelte and drive the full smart-search
lifecycle: POST /api/search/nl via csrfFetch, loading/error panels, chip
row, single-select disambiguation, and a transparent empty state. Chip
removal and disambiguation selection map the interpretation to keyword
params and re-run via GET (Option A in-page fallback). Mode toggle and
new queries reset prior interpretation.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>