[NREM][Space] Set up testing infrastructure (Vitest + React Testing Library)

[numbers-official]

Summary

The ProofSnap extension has zero test files, no test runner, and no testing libraries in its dependencies. All quality assurance currently relies on manual testing, TypeScript type checking, and AI-assisted code review. This gap was visible in PR #9 where code quality issues (untrimmed inputs, accessibility gaps, security flaws) were caught only during human/Copilot review rather than automated tests.

Evidence

• package.json has no test-related scripts or dependencies
• No __tests__/, *.test.ts, or *.spec.ts files exist anywhere in the codebase
• ScreenshotService.ts has an export comment "for testing (if needed)" but no tests were written
• PR Add headline & caption to ProofSnap extension #9 review identified multiple issues that unit tests would catch: untrimmed whitespace in stored data, missing keyboard accessibility, CORS misconfiguration
• Issue [Feature][Medium] Add CI/CD pipeline, linting, debounced settings, and cleanup improvements #23 covers linting and CI but does not address testing framework setup

Proposed Approach

1. Install Vitest — aligns with the existing Vite build system, zero-config for Vite projects
2. Install React Testing Library — for popup/options UI component tests
3. Add chrome API mocks — stub chrome.storage, chrome.runtime, chrome.tabs for unit tests
4. Create initial test suites for critical paths:
   • StorageService — auth token management, settings persistence
   • IndexedDBService — asset CRUD operations
   • UploadService — queue management, retry logic, error recovery
   • AuthService — login/logout flows, token handling
5. Add test script to package.json
6. Integrate into CI — add test step to the release workflow and future PR checks

Priority

High — testing infrastructure is foundational. Without it, every PR relies entirely on manual verification, increasing the risk of regressions as the codebase grows.

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