fix: code review fixes — date calc, input validation, rate limits, shared components

- Fix monthly_reset_at crash when billing anchor day exceeds next month's length
- Add environment_tags sanitization (max 20 tags, 100 chars each) to prevent prompt injection
- Add @limiter.limit("10/minute") rate limiting to all AI endpoints
- Use getTreeNavigatePath() routing helper instead of hardcoded paths
- Extract shared CreateFlowDropdown component from QuickStartPage and TreeLibraryPage
- Clear useCachedQuota on logout to prevent stale data across user sessions
- Add useRef guard to scaffold useEffect to prevent potential double-fire
- Use node.id as React key instead of array index in BranchDetailView
- Remove redundant dead logic in ai_tree_validator

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/store/authStore.ts

@@ -3,6 +3,7 @@ import { persist } from 'zustand/middleware'
 import type { User, Token, UserCreate, UserLogin, Account, SubscriptionDetails } from '@/types'
 import { authApi } from '@/api/auth'
 import { apiClient } from '@/api/client'
+import { clearCachedQuota } from '@/hooks/useCachedQuota'
 
 interface AuthState {
   user: User | null
@@ -79,6 +80,7 @@ export const useAuthStore = create<AuthState>()(
         } finally {
           localStorage.removeItem('access_token')
           localStorage.removeItem('refresh_token')
+          clearCachedQuota()
           set({ user: null, token: null, account: null, subscription: null, isAuthenticated: false, error: null })
         }
       },