Reading activity grid in Settings: months, weeks, days, hours with drill-down

Active reader time is now kept per user in small UTC buckets (6 minutes for
120 days, then hours, dropped after three years), seeded once from the visits
the clock still remembers. Settings -> Reading activity shows them as a grid
whose cells glow with their total and carry a start-to-end strip; month/week
cells open their days, days open their hours. Finished chapters come from
progress and work without the clock.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/settings-navigation.md

@@ -27,7 +27,7 @@ heading when switching within Settings.
 
 ## Routes and ownership
 
-Personal routes: `settings.html#user/reading`, `#user/reading-time`,
+Personal routes: `settings.html#user/reading`, `#user/reading-time`, `#user/activity`,
 `#user/notifications`, `#user/account`.
 
 Admin routes: `settings.html#admin/scraper`, `#admin/library`, `#admin/users`.
@@ -63,6 +63,8 @@ Service health also holds the **Backups** card: built-in backup status (last run
 size, duration, error), **Back up now**, and the daily schedule and retention form,
 whose draft survives the page's polling. See [backup.md](backup.md).
 
+Reading activity is a grid of months, weeks, days or hours (`public/activity.js`). Each cell glows with its total and holds a strip from the period's start (left) to its end (right): days in a month, quarter days in a week, hours in a day, 6 minutes in an hour. Clicking a month or week opens its days (the week is marked), a day opens its hours; hover, keyboard focus or a first tap shows details. It measures reading time (needs the Active Reading Clock) or finished chapters (from progress, always available).
+
 Reading-time collection controls, export and deletion live under Reading time &
 privacy; reader layout controls stay under Reading. Rescan controls and user
 administration are separate. Server & library describes existing maintenance,