feat: Add Step Library API foundation (Phase 1: B.1-B.3, B.7)

Implements foundational types and API clients for Step Library:

Task B.3 - TypeScript Types:
- Created types/step.ts with comprehensive interfaces
- Step, StepListItem, StepCategory types
- StepContent with instructions, help_text, commands
- StepListParams for filtering/sorting
- Rating and Review types
- StepCreate/StepUpdate DTOs

Task B.1 - Steps API Client:
- Created api/steps.ts following existing patterns
- CRUD operations (list, get, create, update, delete)
- Search endpoint with query
- Popular tags endpoint
- Rating/review operations (rate, updateRating, deleteRating, getReviews)

Task B.2 - Step Categories API Client:
- Created api/stepCategories.ts
- List and get operations for categories

Task B.7 - Update API Index:
- Exported stepsApi and stepCategoriesApi
- Available for import from '@/api'

Phase 1 foundation complete. Ready for Phase 2 (UI components).
Build tested successfully.

Related: Issue #10

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/api/index.ts

@@ -6,3 +6,5 @@ export { default as inviteApi } from './invite'
 export { default as tagsApi } from './tags'
 export { default as categoriesApi } from './categories'
 export { default as foldersApi } from './folders'
+export { default as stepsApi } from './steps'
+export { default as stepCategoriesApi } from './stepCategories'