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/stepCategories.ts

@@ -0,0 +1,16 @@
+import apiClient from './client'
+import type { StepCategory } from '@/types/step'
+
+export const stepCategoriesApi = {
+  async list(): Promise<StepCategory[]> {
+    const response = await apiClient.get<StepCategory[]>('/step-categories')
+    return response.data
+  },
+
+  async get(id: string): Promise<StepCategory> {
+    const response = await apiClient.get<StepCategory>(`/step-categories/${id}`)
+    return response.data
+  }
+}
+
+export default stepCategoriesApi