[P0] Stabilize the Vitest environment for pdfjs/react-pdf

[nkanf-dev]

Summary

Fix the unit/UI test environment so pdfjs-dist and react-pdf dependencies do not crash the suite in jsdom.

Why

The current test environment fails with DOMMatrix is not defined, preventing the suite from validating actual logic changes.

Current evidence

• npm run test:unit and UI tests fail before assertions run.
• src/services/PDFService.ts and src/components/reader/ReaderViewport.tsx import PDF dependencies directly in test contexts.

Scope

• Add the required polyfills or targeted mocks for DOMMatrix and related PDF primitives.
• Isolate rendering-heavy dependencies so store/service tests do not load browser-only code unnecessarily.
• Document the chosen strategy in test setup.
• Keep the fix minimal and maintainable.

Acceptance criteria

• Unit and UI test suites start reliably in jsdom.
• PDF-related modules no longer crash test initialization.
• The setup is documented and reproducible on CI.

Out of scope

• Replacing react-pdf or pdfjs-dist entirely.