fix: add missing translations and fix BackupHistory useTranslation error

- Add missing useTranslation hook in BackupHistory.jsx
- Add missing translation keys:
  - backup.dashboard.health.title
  - backup.dashboard.coverage.title
  - backup.dashboard.stats.noBackupsYet
  - backup.configuration.enableBackupHelp
  - backup.configuration.schedule.options.*
  - backup.configuration.messages.*
  - backup.dashboard.noDestinationSet
  - Fix health message keys to match component usage
- Update German translations with same missing keys
- Fix runtime error preventing access to backup history page

🤖 Generated with Claude Code

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

frontend/src/components/admin/BackupHistory.jsx

@@ -1,4 +1,5 @@
 import React, { useState } from 'react';
+import { useTranslation } from 'react-i18next';
 import {
   Download,
   Eye,
@@ -41,6 +42,7 @@ const formatBytes = (bytes) => {
 };
 
 export const BackupHistory = () => {
+  const { t } = useTranslation();
   const [expandedRows, setExpandedRows] = useState(new Set());
   const [searchTerm, setSearchTerm] = useState('');
   const [filterStatus, setFilterStatus] = useState('all');