Add frontend testing guide

[Mayreck]

Summary

• Add docs/TESTING_GUIDE.md covering Vitest, React Testing Library, happy-dom, coverage, fetch mocks, Freighter mocks, and fake timers
• Link the guide from README.md and DEVELOPER_GUIDE.md
• Add executable examples in src/lib/__tests__/testingGuideExamples.test.tsx

Validation

• npx vitest run src/lib/__tests__/testingGuideExamples.test.tsx
• npx eslint src/lib/__tests__/testingGuideExamples.test.tsx

Baseline notes

• Existing repo-wide npm run lint and npx tsc --noEmit remain blocked by unrelated baseline syntax/lint issues noted in PR Add RecentAttestationsPanel test coverage #663.

Closes #658.

[vercel]

@Mayreck is attempting to deploy a commit to the 1nonly's projects Team on Vercel.

A member of the Team first needs to authorize it.

[GBOYEE]

I'd like to work on this.

Approach:

• I'll write the test cases and ensure they pass with full coverage
• Verify with existing tests + add new ones if needed

Estimated effort: ~1-2 hours. PR incoming shortly.

[GBOYEE]

I'd like to work on this.

Approach:

• I'll write the test cases and ensure they pass with full coverage
• Verify with existing tests + add new ones if needed

Estimated effort: ~1-2 hours. PR incoming shortly.