Preserve date offset in meal logging flow

Adds session storage of the 'offset' parameter to ensure the correct date is used when adding meals and redirecting to the daily log. Updates the add_meal and user routes to handle and validate the offset, improving navigation and data consistency for users logging meals on different days.

application/user/routes.py

@@ -5,6 +5,7 @@ from flask import (
     render_template,
     flash,
     abort,
+    session,
 )
 from flask_login import current_user
 from application import db
@@ -76,6 +77,7 @@ def daily_log(offset: int = 0):
         abort(400)  # or handle invalid input
     today = datetime.now(timezone.utc).date()
     day = today + timedelta(days=offset)
+    session["offset"] = offset
     logs_today = current_user.food_logs.filter_by(date_=day).all()
     logs = [[], [], [], []]
     calories: float = 0