feat(shop): implement CSRF protection and improve UI/UX

- Add CSRF token validation to the shop purchase process to prevent cross-site request forgery.
- Implement a unique constraint on `UserShopItem` to prevent duplicate purchases of the same item.
- Refactor the shop template with a modern, responsive grid layout and improved visual feedback for owned items.
- Enhance CSS with better dark/light mode support, including improved navbar styling and scrollbar customization.
- Add `.env_example` and update documentation for environment variable setup.
- Integrate `python-dotenv` for environment variable management.
- Improve logging configuration for the application.
- Update `.gitignore` to include `venv/` and `.env`.

Co-authored-by: Copilot <copilot@github.com>

models.py

@@ -119,6 +119,8 @@ class UserShopItem(db.Model):
     item_id = db.Column(db.Integer, db.ForeignKey('shop_item.id'))
     bought_at = db.Column(db.DateTime, default=datetime.now)
     item = db.relationship('ShopItem')
+    
+    __table_args__ = (db.UniqueConstraint('user_id', 'item_id', name='unique_user_item_purchase'),)
 
 class SupportRequest(db.Model):
     id = db.Column(db.Integer, primary_key=True)