feat: migrate frontend from Vue 3 to React 18 with TanStack ecosystem

- Complete rewrite of frontend using React 18 + TypeScript in strict mode
- Implement TanStack Router for file-based routing matching URL structure
- Use TanStack Query for server state management with smart caching
- Replace Pinia stores with React Context API for auth and UI state
- Adopt Tailwind CSS + shadcn/ui components for consistent styling
- Switch from pnpm to Bun for faster package management and builds
- Configure Vite to support React, TypeScript, and modern tooling
- Create frontend.go with Go embed package for embedding dist/ in binary
- Implement comprehensive TypeScript interfaces (strict mode, no 'any' types)
- Add dark mode support throughout with Tailwind CSS dark: classes
- Set up i18n infrastructure (English translations included)
- Remove all Vue 3 code, components, stores, CSS, and assets
- Includes 18 new files with ~2000 lines of production-ready code

frontend/src/context/AuthContext.tsx

@@ -0,0 +1,104 @@
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useState,
+  ReactNode,
+} from "react";
+import type { IUser } from "@/types";
+import { auth as authAPI } from "@/api";
+
+interface AuthContextType {
+  user: IUser | null;
+  isAuthenticated: boolean;
+  isLoading: boolean;
+  error: string | null;
+  login: (username: string, password: string) => Promise<void>;
+  logout: () => Promise<void>;
+}
+
+const AuthContext = createContext<AuthContextType | undefined>(undefined);
+
+export function AuthProvider({ children }: { children: ReactNode }) {
+  const [user, setUser] = useState<IUser | null>(null);
+  const [isLoading, setIsLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+
+  // Initialize auth state on mount
+  useEffect(() => {
+    async function initAuth() {
+      try {
+        const currentUser = await authAPI.getCurrentUser();
+        setUser(currentUser);
+        setError(null);
+      } catch (err) {
+        setUser(null);
+        // Not an error if not authenticated — just not logged in
+      } finally {
+        setIsLoading(false);
+      }
+    }
+
+    initAuth();
+
+    // Listen for logout events from other tabs/contexts
+    const handleLogout = () => {
+      setUser(null);
+    };
+
+    window.addEventListener("auth:logout", handleLogout);
+    return () => window.removeEventListener("auth:logout", handleLogout);
+  }, []);
+
+  const login = async (username: string, password: string) => {
+    try {
+      setIsLoading(true);
+      setError(null);
+      await authAPI.login(username, password);
+      const currentUser = await authAPI.getCurrentUser();
+      setUser(currentUser);
+    } catch (err) {
+      const message =
+        err instanceof Error ? err.message : "Login failed";
+      setError(message);
+      throw err;
+    } finally {
+      setIsLoading(false);
+    }
+  };
+
+  const logout = async () => {
+    try {
+      await authAPI.logout();
+      setUser(null);
+      setError(null);
+    } catch (err) {
+      console.error("Logout error:", err);
+      // Clear local state even if logout fails
+      setUser(null);
+    }
+  };
+
+  return (
+    <AuthContext.Provider
+      value={{
+        user,
+        isAuthenticated: !!user,
+        isLoading,
+        error,
+        login,
+        logout,
+      }}
+    >
+      {children}
+    </AuthContext.Provider>
+  );
+}
+
+export function useAuth() {
+  const context = useContext(AuthContext);
+  if (!context) {
+    throw new Error("useAuth must be used within AuthProvider");
+  }
+  return context;
+}