feat: add configurable logging level for backend and Mini App

- Introduced a new `LOG_LEVEL` configuration option in the `.env.example` file to allow users to set the logging level (DEBUG, INFO, WARNING, ERROR).
- Updated the `Settings` class to include the `log_level` attribute, normalizing its value to ensure valid logging levels are used.
- Modified the logging setup in `run.py` to utilize the configured log level, enhancing flexibility in log management.
- Enhanced the Mini App to include the logging level in the JavaScript configuration, allowing for consistent logging behavior across the application.
- Added a new `logger.js` module for frontend logging, implementing level-based filtering and console delegation.
- Included unit tests for the new logger functionality to ensure proper behavior and level handling.

webapp/css/calendar.css

@@ -86,11 +86,6 @@
   padding-bottom: 12px;
   margin-bottom: 4px;
   touch-action: pan-y;
-  transition: box-shadow var(--transition-fast) ease-out;
-}
-
-.calendar-sticky.is-scrolled {
-  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
 }
 
 .calendar {