Add unit and integration tests for OpenDrop core flows

OpenDrop currently has minimal or no automated test coverage for its core functionality, including file selection, WebRTC connection setup, data channel messaging, and file transfer completion handling. This issue tracks adding a structured test suite (unit and, where possible, integration tests) to improve reliability and prevent regressions.

### Acceptance criteria:
- A test runner is configured and can be run with a single command (e.g., npm test or pnpm test).
- Core file transfer utilities and signaling helpers have test coverage with clear, deterministic assertions.
- Tests run successfully in CI for pull requests touching relevant code paths.
- README updated with a short “Running tests” section describing prerequisites and commands.