Fix issues from QA review: hash upgrade ordering, interceptor scope, guard

- W-01: Move is_active check before hash upgrade so disabled accounts
  don't get their password hash silently mutated on rejected login
- W-02: Narrow interceptor exclusion to specific auth endpoints instead
  of blanket /auth/* prefix (future-proofs against new auth routes)
- W-03: Add null guard on optimistic setQueryData to handle undefined
  cache gracefully instead of spreading undefined
- S-01: Clear loginError when switching from register back to login mode
- S-03: Add detail dict to auth.login_blocked_inactive audit event

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

backend/app/routers/auth.py

@@ -333,19 +333,20 @@ async def login(
         await db.commit()
         raise HTTPException(status_code=401, detail="Invalid username or password")
 
-    if new_hash:
-        user.password_hash = new_hash
-
     # Block disabled accounts — checked AFTER password verification to avoid
     # leaking account-state info, and BEFORE _record_successful_login so
     # last_login_at and lockout counters are not reset for inactive users.
     if not user.is_active:
         await log_audit_event(
-            db, action="auth.login_blocked_inactive", actor_id=user.id, ip=client_ip,
+            db, action="auth.login_blocked_inactive", actor_id=user.id,
+            detail={"reason": "account_disabled"}, ip=client_ip,
         )
         await db.commit()
         raise HTTPException(status_code=403, detail="Account is disabled. Contact an administrator.")
 
+    if new_hash:
+        user.password_hash = new_hash
+
     await _record_successful_login(db, user)
 
     # SEC-03: MFA enforcement — block login entirely until MFA setup completes

frontend/src/components/auth/LockScreen.tsx

@@ -641,6 +641,7 @@ export default function LockScreen() {
               setUsername('');
               setPassword('');
               setConfirmPassword('');
+              setLoginError(null);
             }}
             className="text-xs text-muted-foreground hover:text-foreground transition-colors"
           >

frontend/src/hooks/useAuth.ts

@@ -36,10 +36,10 @@ export function useAuth() {
         setMfaSetupRequired(false);
         // Optimistically mark authenticated to prevent form flash during refetch
         if ('authenticated' in data && data.authenticated && !('must_change_password' in data && data.must_change_password)) {
-          queryClient.setQueryData(['auth'], (old: AuthStatus | undefined) => ({
-            ...old!,
-            authenticated: true,
-          }));
+          queryClient.setQueryData(['auth'], (old: AuthStatus | undefined) => {
+            if (!old) return old; // let invalidateQueries handle it
+            return { ...old, authenticated: true };
+          });
         }
         queryClient.invalidateQueries({ queryKey: ['auth'] });
       }

frontend/src/lib/api.ts

@@ -15,7 +15,8 @@ api.interceptors.response.use(
     if (error.response?.status === 401) {
       const url = error.config?.url || '';
       // Don't redirect on auth endpoints — they legitimately return 401
-      if (!url.startsWith('/auth/')) {
+      const authEndpoints = ['/auth/login', '/auth/register', '/auth/setup', '/auth/verify-password', '/auth/change-password'];
+      if (!authEndpoints.some(ep => url.startsWith(ep))) {
         window.location.href = '/login';
       }
     }