frontend: add a focused unit and component test foundation

[WalksWithASwagger]

Summary

Add a focused frontend unit and component regression-test foundation around the portal's highest-risk pure behavior.

Context

• Blocked by frontend: remove unreachable UI scaffolding and dead dependencies #319 to serialize frontend/package.json and frontend/package-lock.json.
• Roadmap phase: Phase 1 - polish and stability
• Relevant files: frontend/package.json, frontend/package-lock.json, new Vitest setup/configuration, focused source helpers/components, root package.json, and CI only if the tests cannot be folded into frontend:verify
• Current behavior: the frontend relies on type checking, build, lint, and coarse portal E2E with no unit/component test framework.
• Desired behavior: risky normalization, serialization, and request-state behavior has fast hermetic regression coverage.

Implementation Notes

• Use Vitest, jsdom, and React Testing Library in the existing Vite stack.
• Extract only the minimum pure seams required for testing; do not refactor the full Generate route.
• Mock fetch and filesystem/browser boundaries.
• Keep all tests offline and fixture-driven.

Acceptance Criteria

• frontend:test runs a hermetic Vitest suite.
• Tests cover generation request/error normalization and result/reference URL handling.
• Tests cover library rating/totals normalization and CSV/manifest escaping.
• At least one stateful component renders success and error states.
• CI or frontend:verify invokes the new suite exactly once.
• Existing frontend build, lint, type check, and portal E2E remain green.

Tests/Evals

• Use focused behavior assertions rather than full-page snapshots.
• No provider calls, credentials, local outputs, or live servers beyond existing hermetic harnesses.

Verification

• npm --prefix frontend ci
• npm run frontend:test
• npm run frontend:verify
• npm run e2e:portal
• npm test

Agent Instructions

• Use branch codex/issue-<this-issue>-frontend-test-foundation.
• Do not start until frontend: remove unreachable UI scaffolding and dead dependencies #319 is merged and main is green.
• Keep dependency and lockfile changes limited to the selected test stack.
• Do not set a coverage threshold in this issue.

Human Checkpoints

• Stop if useful coverage requires a redesign of the Generate route rather than a small pure extraction.

Out of Scope

• Whole-page snapshots or exhaustive Generate-route coverage.
• Coverage thresholds and visual baseline CI.
• UI redesign or state-management replacement.
• Provider integration tests.

Linear

Not applicable. Rafiki delivery is GitHub-only; do not create or update a Linear issue.