Analytics: seasonal 'typical week' history curve on day and week views

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.

frontend/src/AnalyticsView.vue

@@ -134,7 +134,8 @@ onUnmounted(() => {
 const window = computed(() => rangeWindow(range.value))
 
 // All non-chart stats follow the selected range; the charts keep their own
-// range-specific x windows (week overlays previous weeks aligned to Monday).
+// range-specific x windows (week aligned to Monday, overlaid with the
+// seasonal "typical week" curve).
 const rangeData = computed(() => {
   if (!data.value) return null
   const { t0, t1 } = window.value