Refactor food item flow and barcode handling

Updated routes and templates to improve the process of adding and logging food items by barcode. The add_food_item route now accepts a barcode parameter, and barcode lookups redirect to item creation if not found. The log_food flow now uses session variables for item and meal selection, and the get_item.html template uses fetch to handle barcode lookups and redirects accordingly.

application/user/templates/add_food_item.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 
 {% block content %}
-<form method="POST" action="{{ url_for('user.add_food_item') }}">
+<form method="POST">
     {{ form.hidden_tag() }}
 
     <div class="mb-3">