feat: Add documentation for prerequisites and implement smoke tests for file system extension

[egalvis27]

What is Changed / Added

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• Added a new PR smoke-test workflow to validate file manager extension installation in a matrix of nautilus, nemo, and dolphin on Ubuntu containers:
  • .github/workflows/file-manager-extension-smoke.yml
• Added a dedicated smoke test script for file manager extension lifecycle checks:
  • src/backend/features/file-manager-extension/file-manager-extension-smoke.ts
• Added a new npm command to run the smoke test locally and in CI:
  • smoke:file-manager-extension in package.json
• Updated documentation with Linux prerequisites/setup notes:
  • README.md

Why

• We recently expanded support to multiple file managers, so we need automated protection to quickly detect installation regressions.
• This smoke suite provides the practical "80% coverage" by validating:
  • file manager detection,
  • extension install/uninstall flow,
  • expected asset paths per manager,
  • Dolphin template replacement ({{HOME}}) and executable permissions.
• The approach is lightweight and CI-friendly, giving faster feedback in pull requests without introducing heavier GUI-level end-to-end complexity.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/add-documentation-and-tests

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[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud