Add step library foundation and user preferences (Issues #3, #5, #6, #7)

Issue #3: User Preferences - export format default
- Add userPreferencesStore with localStorage persistence
- Create Settings page with export format dropdown and theme toggle
- SessionDetailPage now uses default export format from preferences

Issue #5: Step Categories - database table and seed data
- Migration 007: step_categories table with team scoping
- Seed 10 default global categories (Citrix/VDI, AD, M365, etc.)
- Full CRUD API at /api/v1/step-categories

Issue #6: Step Library - database schema
- Migration 008: step_library, step_ratings, step_usage_log tables
- Support for decision/action/solution step types
- Visibility levels: private, team, public
- Rating aggregates and usage tracking

Issue #7: Step Library - CRUD API endpoints
- Full CRUD at /api/v1/steps with visibility filtering
- Full-text search endpoint
- Popular tags endpoint
- Rating/review system with verified use tracking

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

backend/app/api/router.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter
-from app.api.endpoints import auth, trees, sessions, invite, categories, tags, folders
+from app.api.endpoints import auth, trees, sessions, invite, categories, tags, folders, step_categories, steps
 
 api_router = APIRouter()
 
@@ -10,3 +10,5 @@ api_router.include_router(invite.router)
 api_router.include_router(categories.router)
 api_router.include_router(tags.router)
 api_router.include_router(folders.router)
+api_router.include_router(step_categories.router)
+api_router.include_router(steps.router)