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>

README.md

@@ -33,12 +33,21 @@ MiniFacebook is a minimalist social network built with [Flask](https://flask.pal
     pip install -r requirments.txt
     ```
 
-3. **Start**
+3. **Setup the .env file**
+
+    ```sh
+    mv .env_example .env
+    ```
+
+    And you can change this file, to your preferred Settings.
+    ⚠️ You need to set up a PORT
+
+4. **Start**
 
     ```sh
     python main.py
     ```
-4. **Optional:**
+5. **Optional:**
 
    Go to routes/example_oauth.py