Add admin check and update FoodItems model

Introduced an admin_required check for all admin routes using Flask-Login's current_user. Updated the FoodItems model to use per-100g nutritional fields and removed unit relationships. Seed script now creates both admin and regular user accounts.

seed.py

@@ -1,7 +1,11 @@
 from application import db, app
-from models import User
+from models import User, FoodItems
 
 with app.app_context():
     User.query.delete()
-    db.session.add(User("admin", "admin"))
+    db.session.add(User(username="admin", password="admin", is_admin=True))
+    db.session.add(User(username="user", password="user", is_admin=False))
+
+    
+    
     db.session.commit()