Refactor login flow to use auth blueprint

Moved login route and logic from app.py to application/auth/routes.py under the 'auth' blueprint. Updated all references to the login route to use 'auth.login'. Added a dedicated login.html template under application/auth/templates. Adjusted login_required utility and default_return logic for consistency.

application/add_meal/routes.py

@@ -28,7 +28,7 @@ bp = Blueprint(
 @bp.before_request
 def login_required():
     if not current_user.is_authenticated:
-        return redirect(url_for("login"))
+        return redirect(url_for("auth.login"))
 
 
 @bp.route("/select_meal/<int:meal_type>", methods=["GET"])