fix(ui): stack failed series below completed in throughput charts

[omarzeineddine-ai]

Problem

Closes #30

Both throughput charts (queue metrics page and the fleet overview chart) render the failed <Area> after completed within the same stackId. Recharts stacks in child order, so failed sits on top of the stack — and with zero failures its red stroke traces exactly along the top edge of the green completed area. The chart reads as all-errors when there are none.

Solution

Render failed first so it stacks at the bottom:

• 0 failed + N completed → a flat red line on the x-axis with the green completed area on top (the exact behavior requested in the issue).
• With failures present, the red band sits at the base and the top boundary of the stack is green (total throughput), which also reads correctly.

Applied to both occurrences of the pattern: packages/core/src/ui/components/metrics/throughput-chart.tsx and packages/core/src/ui/pages/metrics.tsx.

Note: legend/tooltip entries now list Failed before Completed, following the new series order.

Verification

Reproduced against a local dashboard seeded with 60 completed / 0 failed jobs and confirmed the rebuilt UI bundle emits the failed series before completed for both charts. vite/tsup build and tsc --noEmit pass. (biome check has a pre-existing formatting failure on main in an untouched section of metrics.tsx.)

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