feat(i18n): implement dynamic system wording management

Add database schema and API endpoints for system wording keys and values
Replace hardcoded translations in frontend and backend with dynamic messages
Add System Wordings management interface to Admin view

frontend/src/App.vue

@@ -17,7 +17,7 @@ import {
   iconPokemon,
   iconRecipe
 } from './icons';
-import { getCurrentLocale, onLocaleChange, setCurrentLocale } from './i18n';
+import { getCurrentLocale, loadSystemWordings, onLocaleChange, setCurrentLocale } from './i18n';
 import { api, getAuthToken, onAuthTokenChange, setAuthToken, type AuthUser, type Language } from './services/api';
 
 const { t, locale } = useI18n();
@@ -87,12 +87,15 @@ async function loadLanguages() {
     if (!languages.value.some((language) => language.code === getCurrentLocale() && language.enabled)) {
       setCurrentLocale('en');
     }
+
+    await loadSystemWordings(getCurrentLocale());
   } catch {
     // Keep the built-in language list when the API is not ready yet.
   }
 }
 
-function updateLocale(value: string) {
+async function updateLocale(value: string) {
+  await loadSystemWordings(value);
   setCurrentLocale(value);
 }