An app's page shows its last 30 days of p75 load as a sparkline #3202

Home's visitors tile draws 30 days as a sparkline (#3187). An app's visitors section (appVisitorsView in src/web/pages/settings.ts) shows only today's numbers.

Done looks like:

• [ ] The section draws p75 load per day for the last 30 days as an inline SVG sparkline, server-rendered, with no script. It reuses #3187's sparkline helper.
• [ ] A day with no reports is a gap, not a zero. An app with no reports says so in words.
• [ ] A test renders the section from fixture rows and checks the gap.

Files: src/web/pages/settings.ts, src/apps/vitals.ts, a test.

Parked by the crew: the check stayed red through 2 automatic tries (pull #3228 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("design check: nothing added outside the system (#2678) > the tree is no worse than its baseline") and the decider saw no point in another try (pull #3236 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #3228 closed)
2. the check failed ("design check: nothing added outside the system (#2678) > the tree is no worse than its baseline") and the decider saw no point in another try (pull #3236 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split into #3238 (the sparkline helper draws a missing day as a gap, and adds nothing outside the design system) and #3239 (the 30-day p75 load series and the section, blocked by #3238). Both say to run bun test test/design-lint.test.ts before pushing and to leave docs/design/baseline.json alone. That baseline test is what failed pull #3236.