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/auth.test.js

@@ -66,6 +66,28 @@ describe("getInitData", () => {
     window.location = { ...origLocation, hash: "", search: "" };
     expect(getInitData()).toBe("");
   });
+
+  it("returns data from hash query param without double-decoding", () => {
+    window.Telegram = { WebApp: { initData: "" } };
+    delete window.location;
+    window.location = {
+      ...origLocation,
+      hash: "#tgWebAppVersion=6&tgWebAppData=user%3Dname%26hash%3Dabc",
+      search: "",
+    };
+    expect(getInitData()).toBe("user=name&hash=abc");
+  });
+
+  it("returns data from search query param without double-decoding", () => {
+    window.Telegram = { WebApp: { initData: "" } };
+    delete window.location;
+    window.location = {
+      ...origLocation,
+      hash: "",
+      search: "?tgWebAppData=user%3Dname%26hash%3Dabc",
+    };
+    expect(getInitData()).toBe("user=name&hash=abc");
+  });
 });
 
 describe("isLocalhost", () => {