Home's KPI row shows the age of the last good off-box backup, flagged past 24 hours, linking to /backups #2969

Part of #2740. Waits on the spend tile so the two do not conflict in kpi_row.ts.

Done:

• [ ] One more tile through tile(): the age of the last successful off-box backup (e.g. 3 h ago, the exact time in its title), linking to /backups. Past 24 hours, or with no backup at all, it is flagged with the error/warning class the design system already has (find it in the stylesheet; add none), and the aria-label says it is overdue.
• [ ] No new CSS class, colour or inline style; src/tui/ untouched; design lint and audit at baseline.
• [ ] Test: a backup 2 h old is not flagged; one 25 h old is flagged; none at all is flagged and reads never.

Files: src/web/studio/kpi_row.ts, the Kpis data module, test/home-kpi-row.test.ts.