feat: add backend and frontend test coverage with main-branch CI

[siddqamar]

Summary

This PR adds the missing test foundation for both the backend and frontend, and introduces GitHub Actions CI for the main branch.

What Changed

• Added backend test setup with pytest
• Added backend tests for:
  • Analyzer helper functions
  • FastAPI route behavior with mocked scanner/analyzer flows
• Added frontend tests for:
  • Page-level fetch and error handling
  • Copy-to-clipboard behavior
  • CSV export behavior
• Added a GitHub Actions CI workflow to run:
  • Backend tests
  • Frontend tests
  • Frontend production build
• Updated the README with local test commands

Why This Matters

• Closes a major quality gap by providing baseline automated test coverage for both the backend and frontend
• Makes regressions easier to catch before changes reach main
• Ensures pull requests targeting main are validated automatically
• Gives contributors a clear local workflow that matches CI
• Raises the project to a more maintainable open source standard without adding unnecessary process

Scope

CI is intentionally limited to main branch activity:

• Pushes to main
• Pull requests targeting main