feat(i18n): Russian locale and translations in src/locales

- Add Russian UI strings with idiomatic phrasing and plural forms
- Split en/de/fr/nl/zh/nb strings into src/locales/*.ts for maintainability
- Register ru in i18n and Settings language picker
- Add languageRu to all locales; extend English help (supported languages)

Made-with: Cursor

src/pages/Settings.tsx

@@ -811,6 +811,7 @@ export default function Settings() {
                     { value: 'de', label: t('settings.languageDe') },
                     { value: 'zh', label: t('settings.languageZh') },
                     { value: 'nb', label: t('settings.languageNb') },
+                    { value: 'ru', label: t('settings.languageRu') },
                   ]}
                 />
               </div>