feat(i18n): translate missing backup and provider strings

Update Spanish, Russian, Traditional Chinese and Portuguese locales
to replace placeholder "[to be translated]" messages with proper
translations for backup errors and search provider labels.

- Translate backup.file_locked and backup.file_not_found to meaningful
  localized messages so users see actionable error descriptions when a
  restore fails.
- Translate provider-related flags: is_default and set_as_default so
  UI shows correct localized labels for default search provider
  selection.

These changes improve user experience by providing complete,
context-appropriate localized strings instead of untranslated
placeholders.

src/renderer/src/i18n/locales/zh-tw.json

@@ -1310,8 +1310,8 @@
     "availableTools": "可用工具",
     "backup": {
       "file_format": "備份檔案格式錯誤",
-      "file_locked": "[to be translated]:Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
-      "file_not_found": "[to be translated]:Backup file not found or is corrupted"
+      "file_locked": "無法還原：部分檔案正在使用中。請關閉所有知識庫或重新啟動應用程式後重試。",
+      "file_not_found": "備份檔案未找到或已損壞"
     },
     "base64DataTruncated": "Base64 圖片資料已截斷，大小",
     "boundary": {

src/renderer/src/i18n/translate/de-de.json

@@ -1310,8 +1310,8 @@
     "availableTools": "Verfügbare Tools",
     "backup": {
       "file_format": "Backup-Dateiformat fehlerhaft",
-      "file_locked": "[to be translated]:Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
-      "file_not_found": "[to be translated]:Backup file not found or is corrupted"
+      "file_locked": "Kann nicht wiederhergestellt: Einige Dateien sind in Gebrauch. Bitte schließen Sie alle Wissensdatenbanken oder starten Sie die App neu und versuchen Sie es erneut.",
+      "file_not_found": "Sicherungsdatei nicht gefunden oder beschädigt"
     },
     "base64DataTruncated": "Base64-Bilddaten abgeschnitten, Größe",
     "boundary": {

src/renderer/src/i18n/translate/el-gr.json

@@ -1310,8 +1310,8 @@
     "availableTools": "Διαθέσιμα εργαλεία",
     "backup": {
       "file_format": "Λάθος μορφή αρχείου που επιστρέφεται",
-      "file_locked": "[to be translated]:Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
-      "file_not_found": "[to be translated]:Backup file not found or is corrupted"
+      "file_locked": "Δεν είναι δυνατή η επαναφορά: Μερικά αρχεία χρησιμοποιούνται. Παρακαλώ κλείστε όλες τις βάσεις γνώσεων ή επανεκκινήστε την εφαρμογή και δοκιμάστε ξανά.",
+      "file_not_found": "Αντίγραφο ασφαλείας δεν βρέθηκε ή είναι κατεστραμμένο"
     },
     "base64DataTruncated": "Τα δεδομένα εικόνας Base64 έχουν περικοπεί, μέγεθος",
     "boundary": {

src/renderer/src/i18n/translate/es-es.json

@@ -1310,8 +1310,8 @@
     "availableTools": "Herramientas disponibles",
     "backup": {
       "file_format": "Formato de archivo de copia de seguridad incorrecto",
-      "file_locked": "[to be translated]:Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
-      "file_not_found": "[to be translated]:Backup file not found or is corrupted"
+      "file_locked": "No se puede restaurar: Algunos archivos están en uso. Por favor cierre todas las bases de conocimiento o reinicie la aplicación e intente de nuevo.",
+      "file_not_found": "Archivo de respaldo no encontrado o está corrupto"
     },
     "base64DataTruncated": "Datos de imagen Base64 truncados, tamaño",
     "boundary": {

src/renderer/src/i18n/translate/fr-fr.json

@@ -1310,8 +1310,8 @@
     "availableTools": "Outils disponibles",
     "backup": {
       "file_format": "Le format du fichier de sauvegarde est incorrect",
-      "file_locked": "[to be translated]:Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
-      "file_not_found": "[to be translated]:Backup file not found or is corrupted"
+      "file_locked": "Impossible de restaurer : certains fichiers sont utilisés. Veuillez fermer toutes les bases de connaissances ou redémarrer l'application et réessayer.",
+      "file_not_found": "Fichier de sauvegarde introuvable ou corrompu"
     },
     "base64DataTruncated": "Données d'image Base64 tronquées, taille",
     "boundary": {

src/renderer/src/i18n/translate/ja-jp.json

@@ -1310,8 +1310,8 @@
     "availableTools": "利用可能なツール",
     "backup": {
       "file_format": "バックアップファイルの形式エラー",
-      "file_locked": "[to be translated]:Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
-      "file_not_found": "[to be translated]:Backup file not found or is corrupted"
+      "file_locked": "一部のファイルが使用中です。全ナレッジベースを閉じるか、アプリを再起動して再度お試しください。",
+      "file_not_found": "バックアップファイルが見つからないか、壊れています"
     },
     "base64DataTruncated": "Base64画像データが切り捨てられています、サイズ",
     "boundary": {

src/renderer/src/i18n/translate/pt-pt.json

@@ -1310,8 +1310,8 @@
     "availableTools": "Ferramentas disponíveis",
     "backup": {
       "file_format": "Formato do arquivo de backup está incorreto",
-      "file_locked": "[to be translated]:Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
-      "file_not_found": "[to be translated]:Backup file not found or is corrupted"
+      "file_locked": "Não é possível restaurar: Alguns ficheiros estão em uso. Por favor, feche todas as bases de conhecimento ou reinicie a aplicação e tente novamente.",
+      "file_not_found": "Arquivo de backup não encontrado ou está corrompido"
     },
     "base64DataTruncated": "Dados da imagem em Base64 truncados, tamanho",
     "boundary": {

src/renderer/src/i18n/translate/ru-ru.json

@@ -1310,8 +1310,8 @@
     "availableTools": "Доступные инструменты",
     "backup": {
       "file_format": "Ошибка формата файла резервной копии",
-      "file_locked": "[to be translated]:Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
-      "file_not_found": "[to be translated]:Backup file not found or is corrupted"
+      "file_locked": "Восстановление невозможно: некоторые файлы используются. Закройте все базы знаний или перезапустите приложение и повторите попытку.",
+      "file_not_found": "Файл резервной копии не найден или повреждён"
     },
     "base64DataTruncated": "Данные изображения в формате Base64 усечены, размер",
     "boundary": {