fix(auth): add username validation and account check in login

Ensure case-sensitive username comparison and verify user has at least one account.

app.py

@@ -189,10 +189,21 @@ def login() -> Union[Response, str]:
         response.raise_for_status()
         response_data = response.json()
         if response_data.get("auth") == "Success":
+            # Ensure case-sensitive username comparison
+            if response_data.get("username") != username:
+                return render_template("index.html", error="Invalid username or password. Please try again.")
+            
             session["logged_in"] = True
             session["username"] = response_data.get("username", username)
             session["user_id"] = response_data.get("user_id")
             session["auth_credentials"] = encoded_credentials
+            
+            # Check if the user has at least one account
+            base_url = app.config["BACKEND_URL"]
+            all_accounts = get_user_accounts(base_url, session["auth_credentials"])
+            if not all_accounts:
+                return render_template("index.html", error="No accounts associated with this user.")
+            
             next_url = request.args.get("next")
             if next_url:
                 return redirect(next_url)