Write a frontend testing guide covering Vitest, RTL, and happy-dom conventions

[1nonlypiece]

📌 Description

The project uses Vitest, React Testing Library, happy-dom, and v8 coverage, but
there is no guide documenting how to write tests, mock fetch/Freighter, use
fake timers, or run coverage. Contributors writing the many new test files in
this campaign need a consistent reference.

This issue adds a frontend testing guide capturing the project's conventions and
common mocking patterns.

🎯 Requirements and Context

• Document running tests (pnpm test, test:watch, test:coverage) and the
  coverage threshold expectation.
• Show patterns: mocking fetch, mocking @stellar/freighter-api, fake timers,
  and RTL queries/accessibility-first assertions.
• Document conventions for __tests__ placement and naming.
• Provide a copy-paste component-test and route-test template.

🛠️ Suggested Execution

1. Fork the repo and create a branch

git checkout -b docs/frontend-testing-guide

2. Implement changes

• Add docs/TESTING_GUIDE.md; cross-link from README.md and DEVELOPER_GUIDE.md.
• Add src/lib/__tests__/testingGuideExamples.test.ts that runs the guide's
  example snippets so they cannot rot.
• Validate the example tests pass under the real test runner.

3. Test and commit

• Run pnpm test.
• Cover edge cases: ensure example mocks (fetch/Freighter/timers) actually work.

Example commit message

docs: frontend testing guide with Vitest/RTL conventions

✅ Guidelines

• Minimum 95% test coverage on example helpers.
• Clear, reviewer-friendly documentation.
• Timeframe: 96 hours.

🏷️ Labels

type-documentation · area-frontend · MAYBE REWARDED · GRANTFOX OSS ·
OFFICIAL CAMPAIGN

💬 Community & Support

• Join the CommitLabs contributor Discord to coordinate and get unblocked fast:
  https://discord.gg/WV7tdYkJk
• Introduce yourself before starting so we can avoid duplicate work and pair you
  with a reviewer. Maintainers triage actively and review fast.

[darius-daniel]

@darius-daniel has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi! I'd like to work on this issue.

I have experience with React Testing Library, Vitest, mocking browser APIs, and documenting contributor workflows. I can create a clear TESTING_GUIDE.md covering the project's testing conventions, including fetch mocking, @stellar/freighter-api mocking, fake timers, accessibility-focused RTL patterns, coverage expectations, and test organization.

I'll also add executable example tests so the documented snippets remain validated by the actual test suite, cross-link the guide from existing documentation, and ensure everything passes under the current Vitest configuration.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @darius-daniel to this issue.

[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.