feat: hello page hitting /api/v1/health + CSS-variable palette + sample component test

[constk]

Problem

The frontend quality gate is vacuous without at least one component, one test, and one network call.

Proposed solution

Build App.tsx rendering a status panel that fetches /api/v1/health on mount and shows ok / error. Port the CSS-variable palette from frontend/src/styles/ (theme tokens: --bg-primary, --text-primary, --accent-*). Add App.test.tsx using @testing-library/react to mock the fetch and assert the rendered status.

Acceptance criteria

• App renders without console errors when backend is up.
• Component test passes; covers loading, success, and error paths.
• CSS palette consumed via CSS variables, not hard-coded hex.
• npm run check && npm run test && npm run build exits 0.

Priority rationale

Medium: provides a concrete starting point so users aren't staring at a blank App.tsx.

Depends on

#21, #22

[constk]

Merged in PR #59.