fix(dashboard): show remaining totals in usage donuts

[Daltonganger]

Summary

• use aggregate remaining credits instead of aggregate capacity for the dashboard 5h and weekly usage donut totals
• keep the donut center value aligned with the existing per-account remaining slices and the visible “Remaining” label
• add a focused frontend regression test so the dashboard view does not fall back to capacity totals again

Testing

• bun run test -- src/features/dashboard/utils.test.ts src/features/dashboard/components/usage-donuts.test.tsx
• bun run lint -- src/features/dashboard/components/dashboard-page.tsx src/features/dashboard/utils.ts src/features/dashboard/utils.test.ts
• bun run typecheck