[Tests] Set Up Testing Infrastructure and Tests Folder

[evanautianle]

User Story

As a developer
I want to set up a testing infrastructure with a dedicated tests folder and example tests
So that I can easily write, organize, and run tests to ensure code quality

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Acceptance Criteria (AC)

• A tests/ directory exists at the project root
• Jest and React Testing Library are installed and configured for TypeScript
• A sample test file exists in tests/ and/or as a colocated *.test.ts(x) file
• Test command is documented in README.md
• All tests pass with the test command

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Definition of Done

• AC met
• PR reviewed
• merged to main/feature branch