Add explicit "mimetype" to ensure CSS is handled correctly

[jzohrab]

In issue #623 , @newexploder found and fixed a CSS issue where the files weren't getting loaded correctly. Weird that it happened on his/her machine only (so far), but the fix should probably be applied to the app code as it's very fast.

Borrowing from the notes in that issue:

---

Root Cause

The root cause was an incorrect MIME type being served for the CSS files.

• Expected Content-Type: text/css
• Actual Content-Type served: application/x-css

The Solution

The issue was resolved by adding two lines of code to the application factory file (lute/app_factory.py) to explicitly register the correct MIME type for CSS files within the Python environment.

1. Add the mimetypes import to the top of the file:

   import mimetypes

2. Add the following line after all the imports, before the create_app() function:

   mimetypes.add_type('text/css', '.css')

After adding this code and restarting the server, the CSS is now served with the correct text/css MIME type, and the application's layout renders perfectly.

---

This should be a very fast fix.