feat(docker): support project-specific Dockerfiles

[ASRagab]

Summary

• detect and build project-specific Dockerfiles from .parallel-code/Dockerfile
• pass an explicit project build context into Docker builds instead of inferring from the Dockerfile directory
• track Docker source explicitly (default | project | custom) instead of inferring UI state from image-tag prefixes
• harden Docker image checks for unreadable custom Dockerfiles and clean up the new-task / Docker UI flow
• ignore repo-local .worktrees/** in Electron Vite dev mode so task creation does not trigger renderer reloads

Validation

• npm run compile
• npm run lint
• npm test
• npm run typecheck