fix: complete backup page translations and improve UI

- Fix '0 files' hardcoded string to use translation
- Fix 'local' destination type to show translated name
- Add missing field placeholders for rsync and S3 configurations
- Add missing backup.history.columns.* translations
- Add missing backup.history.filter.* translations
- Add missing backup.history.details.* translations
- Fix backup destination display to use proper translation key
- Replace TestTube icon with Wifi icon for connection testing
- Add all corresponding German translations
- Ensure Backup Health and Coverage titles use translations

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/admin/BackupConfiguration.jsx

@@ -12,7 +12,7 @@ import {
   Info,
   Eye,
   EyeOff,
-  TestTube,
+  Wifi,
   CheckCircle,
   XCircle,
   Loader2,
@@ -386,7 +386,7 @@ export const BackupConfiguration = ({ config, onSave, isSaving }) => {
                   </>
                 ) : (
                   <>
-                    <TestTube className="mr-2 h-4 w-4" />
+                    <Wifi className="mr-2 h-4 w-4" />
                     {t('backup.actions.testConnection')}
                   </>
                 )}