no separate login/register page, use modal throughout

frontend/src/components/AuthModal.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect, useRef, useCallback } from "react";
+import React, { useState, useEffect } from "react";
 import { useAuth } from "../contexts/AuthContext";
 import PasswordStrengthMeter from "./PasswordStrengthMeter";
 import PasswordInput from "./PasswordInput";