feat(status-mix): stacked column chart with ok baseline

[coccyx]

Summary

Two requested changes to the Status mix chart on Service Detail:

1. Chart type → stacked column instead of line. Each column is a 1-minute bucket; segments inside stack bottom-up.
2. Include ok class → the query no longer filters `status_class != "ok"`. The slate baseline shows total request volume and errors stack visibly above it.

After

`frontend-proxy` over the last hour (ok 71%, 504 13%, 503 10%, 500 5%, 4xx 1%):

Implementation

• `src/components/StackedColumnChart.tsx` — new SVG component, reuses `LineChart.module.css` so the shell (title / subtitle / legend / tooltip / empty state) matches sibling panels. Hover tooltip lists segments top-to-bottom matching what the operator sees onscreen, plus a "total" row.
• `src/api/queries.ts` — dropped the `where status_class != "ok"` filter.
• `src/api/types.ts` — added `'ok'` to `StatusCodeClass`, reordered `STATUS_CODE_CLASSES` to define stack order (ok at bottom, errors stacked above).
• `src/routes/ServiceDetailPage.tsx` — swap LineChart → StackedColumnChart for status mix, added the ok color (slate-300).

Test plan

• Open https://main-objective-shirley-sho21r7.cribl-staging.cloud/ → APM → Services → `frontend-proxy`.
• Verify the chart is now stacked columns, not lines.
• Verify the legend includes `ok` (light slate swatch).
• Verify the bottom slate segment dominates each bar (since ok ≫ errors on a healthy service).
• Hover a column — tooltip should show segments top-to-bottom + a "total" row.
• Pick a service with zero errors (e.g. a healthy one). Verify column is solid slate with no error segments.

Pre-merge checks

• ✓ `npm run lint` — 0 errors, 1 pre-existing warning
• ✓ `npx tsc --noEmit` — clean
• ✓ `npm test` — 63/63 passing

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