feat: unify language handling across the application

- Updated the language configuration to use a single source of truth from `DEFAULT_LANGUAGE` for the bot, API, and Mini App, eliminating auto-detection from user settings.
- Refactored the `get_lang` function to always return `DEFAULT_LANGUAGE`, ensuring consistent language usage throughout the application.
- Modified the handling of language in various components, including API responses and UI elements, to reflect the new language management approach.
- Enhanced documentation and comments to clarify the changes in language handling.
- Added unit tests to verify the new language handling behavior and ensure coverage for the updated functionality.

webapp/css/calendar.css

@@ -152,6 +152,19 @@
   border: 1px solid color-mix(in srgb, var(--bg) 50%, transparent);
 }
 
+@media (hover: hover) {
+  .day.holiday:hover {
+    background: linear-gradient(
+      135deg,
+      color-mix(in srgb, var(--accent) 12%, var(--surface)) 0%,
+      color-mix(in srgb, var(--today) 22%, transparent) 100%
+    );
+  }
+  .day.today.holiday:hover {
+    background: color-mix(in srgb, var(--bg) 15%, var(--today));
+  }
+}
+
 .day {
   cursor: pointer;
 }