Port the seasonal.py demo algorithm to JS (analytics/seasonal.js): the
smoothed 5-minute history is folded onto a weekly grid with exponential
age decay (7-day half-life for the time-of-day pattern, 42 days for
per-weekday deviations, shrunk by effective weeks of data), capped at
180 days of history where the weights become negligible.
Week view: the fading past-week overlays are replaced by the full
Monday-first 'Typical week' estimate. Day view gains the same estimate
cut to the rolling 24h window as a smooth muted 'Typical <weekday>'
curve behind the bars. Longer ranges are unchanged.