Add core CI and frontend test coverage

[VolkannSahin]

• Add GitHub Actions CI for all pushes, pull requests, and manual runs.
• Run Python 3.11 dependency checks and backend tests.
• Run Node 24 frontend tests and production builds.
• Add Vitest, jsdom, and React Testing Library.
• Add 56 tests covering APIs, authentication, routes, utilities, and ARCH/VNN submission workflows.
• Require tests by removing --passWithNoTests.
• Cancel outdated CI runs and use read-only workflow permissions.
• Document frontend and backend test commands.