Add new dashboard widget showing storage usage stats:
- Total storage used and file count
- Recent files list with icons
- Link to open Storage app
Backend changes:
- Add GET /api/v1/files/stats endpoint to storage backend
- Returns totalFiles, totalSize, favoriteCount, recentFiles
Frontend changes:
- Add storageService API client
- Add StorageUsageWidget component
- Add i18n translations (de/en)
- Register widget in dashboard types and container
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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