Home's visitors tile shows the last 30 days as a sparkline, linking to its board #3186

Part of #2674. Home's KPI row shows visitors as one number (#3037).

Done:

• [ ] The visitors tile carries a server-rendered inline SVG sparkline of distinct visitors per day for 30 days. It needs no script and has a fixed size, so nothing shifts.
• [ ] The tile links to a board panel of the same page_vital query.
• [ ] The sparkline is admin only and is cached under the KPI row's existing key (docs/CACHING.md, cache 4). A test asserts 30 points, and that a stranger never sees it.

Files: the KPI row's module under src/web/studio/, a test.