Analytics charts: translucent typical fill, last-week tail on week view

Render the seasonal typical estimate as a translucent muted fill (no
stroke, same 0.6 opacity as the data) instead of a grey line, on both day
and week views. Continue the fixed Monday-Monday week view with last
week's curve from the current bucket to the end of the week (secondary
accent, translucent fill), so the chart is useful early in the week and
last week is gradually replaced by the current one.

Gate the typical estimate on history spanning twice the view's full time
(third day / third week on) and the last-week tail on data reaching into
last week at all; the legend shows each entry only when present.

docs/analytics.md

@@ -366,8 +366,17 @@ range is 10 so tiny values such as a single visit are not stretched to a
 fractional scale).
 The week range is aligned to Monday 00:00 UTC (the current week keeps the
 accent color and is truncated at the current bucket, never drawing fake
-zeroes for the future). Both the week and day views overlay a **"typical"
-history curve** in the muted color (`analytics/seasonal.js`, a port of
+zeroes for the future). Since the window is fixed Monday-to-Monday, **last
+week's curve** continues the graph from the current bucket to the end of
+the week in the secondary accent (`--accent2`, translucent fill like the
+current week), so the chart shows useful data
+on Monday too and last week is gradually replaced by the current week;
+the tail is only drawn when the recorded data reaches into last week.
+Both the week and day views overlay a **"typical"
+history estimate** as a muted fill with no stroke, translucent to the same
+degree as the current data — shown only once the history spans twice the
+view's full time (from the third day on the day view, the third week on
+the week view; `analytics/seasonal.js`, a port of
 `seasonal.py`): the whole recorded history is densified to 5-minute bins,
 smoothed with the same Gaussian as the week view, then folded onto a weekly
 grid with exponential decay over age — a 7-day half-life for the average
@@ -381,7 +390,9 @@ estimate as "Typical week" (future included); the day view cuts the rolling
 24-hour window's bins from the same estimate and labels them by the weekday
 ("Typical Saturday"). A compact legend inside the top right of the visits
 chart marks the current data in accent (ISO week label, or a bar specimen
-for "Last 24 hours") and the typical curve on a muted specimen. The week
+for "Last 24 hours"), the previous week's tail on a secondary-accent line
+specimen (week view only), and the typical estimate on a muted fill
+specimen. The week
 view's x labels are weekday names centered at midday UTC, without
 vertical grid
 lines (day boundaries would be misleading in the viewer's timezone). The