feat(frontend): add React dashboard with TypeScript

Add modern React frontend to replace Streamlit dashboard:
- Vite build system with TypeScript
- Tailwind CSS for styling
- Component structure in src/
- Production Dockerfile with nginx
- Development server on port 5173

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/context/AuthContext.tsx

@@ -0,0 +1,81 @@
+import { createContext, useContext, useState, useEffect, ReactNode } from 'react';
+import { authApi, getAccessToken } from '../api/client';
+import type { User } from '../types';
+
+interface AuthContextType {
+  user: User | null;
+  isLoading: boolean;
+  isAuthenticated: boolean;
+  isAdmin: boolean;
+  login: (email: string, password: string) => Promise<void>;
+  register: (email: string, password: string) => Promise<void>;
+  logout: () => void;
+  refreshUser: () => Promise<void>;
+}
+
+const AuthContext = createContext<AuthContextType | undefined>(undefined);
+
+export function AuthProvider({ children }: { children: ReactNode }) {
+  const [user, setUser] = useState<User | null>(null);
+  const [isLoading, setIsLoading] = useState(true);
+
+  const refreshUser = async () => {
+    try {
+      const userData = await authApi.getMe();
+      setUser(userData);
+    } catch {
+      setUser(null);
+    }
+  };
+
+  useEffect(() => {
+    const initAuth = async () => {
+      if (getAccessToken()) {
+        await refreshUser();
+      }
+      setIsLoading(false);
+    };
+    initAuth();
+  }, []);
+
+  const login = async (email: string, password: string) => {
+    await authApi.login(email, password);
+    await refreshUser();
+  };
+
+  const register = async (email: string, password: string) => {
+    await authApi.register(email, password);
+    await authApi.login(email, password);
+    await refreshUser();
+  };
+
+  const logout = () => {
+    authApi.logout();
+    setUser(null);
+  };
+
+  return (
+    <AuthContext.Provider
+      value={{
+        user,
+        isLoading,
+        isAuthenticated: !!user,
+        isAdmin: user?.role === 'admin',
+        login,
+        register,
+        logout,
+        refreshUser,
+      }}
+    >
+      {children}
+    </AuthContext.Provider>
+  );
+}
+
+export function useAuth() {
+  const context = useContext(AuthContext);
+  if (context === undefined) {
+    throw new Error('useAuth must be used within an AuthProvider');
+  }
+  return context;
+}