fix: improve env setup and prevent undefined auth API URL

- Add frontend and backend .env.example files
- Document required environment variables
- Add fallback for VITE_BACKEND_URL in auth pages
- Prevent malformed undefined/api/auth/... requests

Author: vv-verse

.env.example

@@ -0,0 +1,6 @@
+# Backend API URL
+# Local development
+VITE_BACKEND_URL=http://localhost:5000
+
+# Production example
+# VITE_BACKEND_URL=https://your-backend-url.onrender.com

backend/.env.example

@@ -0,0 +1,8 @@
+# MongoDB connection string
+MONGO_URI=mongodb://localhost:27017/github_tracker
+
+# Session secret - use a strong random string
+SESSION_SECRET=your_session_secret_here
+
+# Server port
+PORT=5000

src/pages/Login/Login.tsx

@@ -4,7 +4,7 @@ import { useNavigate, Link } from "react-router-dom";
 import { ThemeContext } from "../../context/ThemeContext";
 import type { ThemeContextType } from "../../context/ThemeContext";
 
-const backendUrl = import.meta.env.VITE_BACKEND_URL;
+const backendUrl = import.meta.env.VITE_BACKEND_URL || '';
 
 interface LoginFormData {
   email: string;

src/pages/Signup/Signup.tsx

@@ -6,7 +6,7 @@ import { User, Mail, Lock, Eye, EyeOff } from "lucide-react";
 import { ThemeContext } from "../../context/ThemeContext";
 import type { ThemeContextType } from "../../context/ThemeContext";
 
-const backendUrl = import.meta.env.VITE_BACKEND_URL;
+const backendUrl = import.meta.env.VITE_BACKEND_URL || '';
 
 interface SignUpFormData {
   username: string;