Fix week overlay alignment, in-plot ISO week legend, shorter charts

- weeklySeries shifts overlaid weeks onto the current week's time axis so
  they overlay inside the plot instead of overflowing left; oldest weeks
  paint first, current week on top
- Legend moved inside the visits chart's top right: current ISO week in
  accent, past weeks as a single muted "Week M" / "Week M–N" specimen
- Past week curves use the muted color instead of faded accent
- Chart height reduced ~30% (180 -> 126)

frontend/src/analytics/chart.js

@@ -10,7 +10,7 @@ import { DAY, HOUR, MIN5, WEEK, mondayUTC } from './time.js'
 import { formatCount } from './format.js'
 
 export const CHART_W = 720
-export const CHART_H = 180
+export const CHART_H = 126
 export const PAD_TOP = 14 // room above the highest point
 export const MARGIN_L = 56 // y tick labels + vertical axis label
 export const MARGIN_B = 24 // x tick labels