fix: division by zero guard, logout redirect, onboarding close button

- auto_trader_risk: skip drawdown check when entryPrice <= 0
- AuthContext: redirect to / on logout
- App.tsx: simplify data page navigation
- BeginnerOnboardingPage: add close button to overlay

web/src/contexts/AuthContext.tsx

@@ -290,6 +290,8 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
     setToken(null)
     localStorage.removeItem('auth_token')
     localStorage.removeItem('auth_user')
+    window.history.pushState({}, '', '/')
+    window.dispatchEvent(new PopStateEvent('popstate'))
   }
 
   return (