Add budget module: FastAPI backend + React frontend

Backend: FastAPI + PostgreSQL with models for accounts, transactions,
and categories. Auto-categorization from merchant patterns, token auth,
CRUD endpoints, and seed data for 16 categories and 4 bank accounts.

Frontend: Login, Dashboard (account balances + recent charges),
Transactions (full CRUD table with search/filter), Cash & Transfers
view. Dark theme with emerald/cyan accents, responsive layout.

Infrastructure: Updated docker-compose for backend + db services,
nginx proxy config for API routing, deploy workflow with secrets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/AuthContext.tsx

@@ -0,0 +1,36 @@
+import { createContext, useContext, useState, useEffect, type ReactNode } from 'react';
+
+interface AuthCtx {
+  isAuthenticated: boolean;
+  logout: () => void;
+  setAuthenticated: (v: boolean) => void;
+}
+
+const AuthContext = createContext<AuthCtx>({
+  isAuthenticated: false,
+  logout: () => {},
+  setAuthenticated: () => {},
+});
+
+export function AuthProvider({ children }: { children: ReactNode }) {
+  const [isAuthenticated, setAuthenticated] = useState(!!localStorage.getItem('token'));
+
+  useEffect(() => {
+    const check = () => setAuthenticated(!!localStorage.getItem('token'));
+    window.addEventListener('storage', check);
+    return () => window.removeEventListener('storage', check);
+  }, []);
+
+  const logout = () => {
+    localStorage.removeItem('token');
+    setAuthenticated(false);
+  };
+
+  return (
+    <AuthContext.Provider value={{ isAuthenticated, logout, setAuthenticated }}>
+      {children}
+    </AuthContext.Provider>
+  );
+}
+
+export const useAuth = () => useContext(AuthContext);