feat(frontend): establish v0.5 application foundation

[apantaz]

Summary

Establishes the v0.5 frontend foundation using React, TypeScript, and Vite.

Changes

• Added the responsive WatchPulse application shell
• Added a typed FastAPI client and catalog-readiness display
• Added loading and API/catalog error states
• Added environment-controlled frontend CORS
• Added frontend tests, linting, type checking, and production build
• Added frontend Makefile commands and a dedicated CI job
• Added Node.js 22 configuration and setup documentation
• Documented the frontend decision in ADR-018
• Updated the README and v0.5 roadmap status

Validation

• 2 frontend tests pass
• Frontend lint, typecheck, and production build pass
• 104 Python tests pass
• Pre-commit and pre-push hooks pass