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/js/dom.js

@@ -67,7 +67,7 @@ export const state = {
   /** @type {ReturnType<typeof setInterval>|null} */
   todayRefreshInterval: null,
   /** @type {'ru'|'en'} */
-  lang: "ru",
+  lang: "en",
   /** One-time bind flag for sticky scroll shadow listener. */
   stickyScrollBound: false,
   /** One-time bind flag for calendar (info button) hint document listeners. */