Fix Vite proxy inside Docker and add success pages

- vite.config.ts: proxy target via VITE_API_TARGET env var (falls back to localhost)
- docker-compose.dev.yml: set VITE_API_TARGET=http://backend:8000
- Add /login-success and /register-success placeholder pages
- Show real API error messages in login/register forms

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

frontend/src/App.tsx

@@ -2,6 +2,8 @@ import { Routes, Route, Navigate } from "react-router-dom";
 import LoginPage from "./pages/LoginPage";
 import RegisterPage from "./pages/RegisterPage";
 import DashboardPage from "./pages/DashboardPage";
+import LoginSuccessPage from "./pages/LoginSuccessPage";
+import RegisterSuccessPage from "./pages/RegisterSuccessPage";
 import { useAuth } from "./hooks/useAuth";
 
 function PrivateRoute({ children }: { children: React.ReactNode }) {
@@ -14,6 +16,15 @@ export default function App() {
     <Routes>
       <Route path="/login" element={<LoginPage />} />
       <Route path="/register" element={<RegisterPage />} />
+      <Route path="/register-success" element={<RegisterSuccessPage />} />
+      <Route
+        path="/login-success"
+        element={
+          <PrivateRoute>
+            <LoginSuccessPage />
+          </PrivateRoute>
+        }
+      />
       <Route
         path="/"
         element={