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.

tests/test_app.py

@@ -23,6 +23,41 @@ def test_health(client):
     assert r.json() == {"status": "ok"}
 
 
+def test_health_has_vary_accept_language(client):
+    """NoCacheStaticMiddleware adds Vary: Accept-Language to all responses."""
+    r = client.get("/health")
+    assert r.status_code == 200
+    assert "accept-language" in r.headers.get("vary", "").lower()
+
+
+def test_app_static_has_no_store_and_vary(client):
+    """Static files under /app get Cache-Control: no-store and Vary: Accept-Language."""
+    r = client.get("/app/")
+    if r.status_code != 200:
+        r = client.get("/app")
+    assert r.status_code == 200, "webapp static mount should serve index at /app or /app/"
+    assert r.headers.get("cache-control") == "no-store"
+    assert "accept-language" in r.headers.get("vary", "").lower()
+
+
+def test_app_js_has_no_store(client):
+    """JS and all static under /app get Cache-Control: no-store."""
+    r = client.get("/app/js/main.js")
+    assert r.status_code == 200
+    assert r.headers.get("cache-control") == "no-store"
+
+
+def test_app_config_js_returns_lang_from_default_language(client):
+    """GET /app/config.js returns JS setting window.__DT_LANG from config.DEFAULT_LANGUAGE."""
+    r = client.get("/app/config.js")
+    assert r.status_code == 200
+    assert r.headers.get("content-type", "").startswith("application/javascript")
+    assert r.headers.get("cache-control") == "no-store"
+    body = r.text
+    assert "window.__DT_LANG" in body
+    assert config.DEFAULT_LANGUAGE in body
+
+
 def test_duties_invalid_date_format(client):
     r = client.get("/api/duties", params={"from": "2025-01-01", "to": "invalid"})
     assert r.status_code == 400