Create API Test Collection Builder

## Description\nBuild a Postman-like interface for creating and managing API test collections.\n\n**Acceptance Criteria:**\n- Create/Edit test collections\n- Save and share collections\n- Environment variables support\n- Test automation

The issue names no files, tests, or entry points. Start by mapping the repository's TypeScript API structure and existing test or collection-related functionality, then clarify the design and scope for collection creation, sharing, environment variables, and automation before implementation; done means all listed acceptance criteria work.

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