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/api.js

@@ -17,7 +17,7 @@ import { t } from "./i18n.js";
 export function buildFetchOptions(initData, externalSignal) {
   const headers = {};
   if (initData) headers["X-Telegram-Init-Data"] = initData;
-  headers["Accept-Language"] = state.lang || "ru";
+  headers["Accept-Language"] = state.lang || "en";
   const controller = new AbortController();
   const timeoutId = setTimeout(() => controller.abort(), FETCH_TIMEOUT_MS);
   const onAbort = () => controller.abort();