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/architecture.md

@@ -210,6 +210,7 @@ All requests and responses are JSON. POSTs require `Content-Type: application/js
 | `GET /api/progress/latest?m=` | user | The manga's newest saved position (`latest`: `t, w, c, label, v, p, n, done, continue`) or `null` |
 | `POST /api/reading/pulse` `{m, c, v, session, seq, …deltas}` | user | When opted in, idempotently merge a clamped active-time pulse, verified page coverage and input totals under a single-reader lease |
 | `GET /api/reading/export` | user | Export the user's private derived reading-time summary without raw input or page-exposure diagnostics |
+| `GET /api/reading/activity` | user | The user's reading activity for the Settings grid: active reader time in UTC buckets `[start, size, ms, [[mangaIndex, ms]...]]` (6 minutes for the last 120 days, hours up to three years), chapter completions `[doneAt, mangaIndex, label]`, manga titles and whether the clock is on. The browser groups them into local months, weeks, days and hours (`public/activity-core.js`) |
 | `POST /api/reading/delete` `{m}` or `{all:true,password}` | user | Delete one manga's reading history, or all history after password confirmation, without changing normal reading progress |
 | `POST /api/unread` `{m, c}` | user | Forget progress for a chapter |
 | `POST /api/vote` `{m, c, v, value: 1\|-1\|0}` | user | Vote a version; returns the re-ranked versions |