The clock says what changed since the agent last looked: dates moved, marked done or newly overdue #318

One sentence: on opening the clock the agent sees one quiet line of what moved since their last visit, so a sales-meeting demo shows the app tracking a live book.

Done:

• [ ] The last visit time is kept per user under FORGE_APP_DATA (bun:sqlite), updated on a clock page view.
• [ ] A line above the clock reads e.g. "Since yesterday: 1 date moved, 2 marked done, 1 newly overdue", built from the existing activity log; absent when nothing changed and on first visit.
• [ ] Output escaped; no layout shift (line is server-rendered).
• [ ] Tests: counts correct, absent when empty; fails without the feature. README tour mentions it.

Files: src/activity.ts, a new small module (e.g. src/since.ts), src/routes.ts, src/pages/, README.md, tests. Risks: reuse activity.ts rather than a second log; no contract cell changes. Suggested: mason.