refactor(utils): consolidate date and file size formatting
Extracts formatting logic into dedicated utility functions to improve
code maintainability and consistency across components.
Changes:
- Consolidate date formatting functions (formatRelativeTime, etc.)
- Consolidate file size formatting into single formatFileSize utility
- Fix file size calculation errors
- Update UI components to use new utilities
- Change "ASAP" to "Waiting" for unscheduled queue items
Tests:
- Add comprehensive unit tests for date utilities
- Add unit tests for file size formatting
- Add checkbox selection for individual files
- Add Select All / Deselect All buttons
- Download single file directly, multiple files as ZIP archive
- Add POST /printers/{id}/files/download-zip endpoint
- Clear selection after successful download
- Support bulk delete for multiple files
- Update docs (changelog, wiki, website)
Closes#144