refactor: apply React best practices and remove mock data from deployment UI

- Fix barrel imports: import directly from source files in deployments-page,
  step-provider, and step-attach-flows
- Replace useEffect auto-select with derived effectiveFlowId in step-attach-flows
- Fix async state init bug for environmentTab using useRef guard pattern
- Fix stale closure in handleAddEnvVar with functional setState
- Wrap useState initial value in lazy initializer for envVars
- Wrap all 8 handlers in useCallback; wrap panel components in memo()
- Remove mock-data.ts; move PROVIDERS constant inline to step-provider
- Drop MOCK_CONNECTIONS (was empty array) from context
- Simplify step-provider UI: remove provider selection radio group since
  only one provider exists; show watsonx Orchestrate as a static display