refactor: item 29 cleanup

F16 F17 F18 F19 F20 F21: import polling backs off and pauses in a hidden tab, one date format module, fake timers in the import tests, error codes instead of raw messages, one safe storage wrapper, dead code.

src/utils/settings.js

@@ -1,4 +1,5 @@
 import {apiFetch} from './api.js';
+import {storage} from './storage.js';
 
 export const LANGUAGES = ['en', 'de'];
 
@@ -6,23 +7,13 @@ const LANGUAGE_STORAGE_KEY = 'tbp_language';
 export const REQUEST_TIMEOUT_MS = 10000;
 
 export const readCachedLanguage = () => {
-    try {
-        const stored = localStorage.getItem(LANGUAGE_STORAGE_KEY);
-        return LANGUAGES.includes(stored) ? stored : null;
-    } catch {
-        return null;
-    }
+    const stored = storage.get(LANGUAGE_STORAGE_KEY);
+    return LANGUAGES.includes(stored) ? stored : null;
 };
 
-export const cacheLanguage = (language) => {
-    try {
-        localStorage.setItem(LANGUAGE_STORAGE_KEY, language);
-    } catch {
-        // Private mode or a full storage: the language is only not cached.
-    }
-};
+export const cacheLanguage = (language) => storage.set(LANGUAGE_STORAGE_KEY, language);
 
-export const ME_URL = () => `${import.meta.env.VITE_API_URL}/api/users/me?v=1`;
+export const ME_URL = () => `${import.meta.env.VITE_API_URL}/api/users/me`;
 
 /**
  * GET /api/users/me on Auth. The caller handles the response status.