Fix changes requested (#101)

- Fix code formatting issues.
- Document `Presence`.
- Improve `Presence.__lt__()` performance by
  defining a dict outside the method.
- Make presence ball radius relative to uiScale
  and configurable from theme.

src/backend/backend.py

@@ -82,6 +82,9 @@ class Backend:
             we managed. Every client is logged to one matrix account.
 
         media_cache: A matrix media cache for downloaded files.
+
+        presences: A `{user_id: Presence}` dict for storing presence info about
+            matrix users registered on Mirage.
     """
 
     def __init__(self) -> None: