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>

public/settings-nav.js

@@ -5,6 +5,7 @@ const SETTINGS_SECTIONS = [
   { id: 'user/reading', group: 'User', title: 'Reading', hint: 'Reader modes, page spacing and zoom.' },
   { id: 'user/reading-hours', group: 'User', title: 'Reading hours', hint: 'When this server allows chapter reading.' },
   { id: 'user/reading-time', group: 'User', title: 'Reading time & privacy', hint: 'Your clock, Reading Story and personal data.' },
+  { id: 'user/activity', group: 'User', title: 'Reading activity', hint: 'When you read: by month, week, day or hour. Private to your account.' },
   { id: 'user/shortcuts', group: 'User', title: 'Keyboard shortcuts', hint: 'Every key the reader understands. Press ? in the reader for the same list.' },
   { id: 'user/notifications', group: 'User', title: 'Notifications', hint: 'Chapter alerts and your optional webhook.' },
   { id: 'user/account', group: 'User', title: 'Account', hint: 'Your account and password.' },