- Add ButtonConfigActivity for customizing main screen buttons with
drag-and-drop reordering and individual size options (S/M/L)
- Move storage settings to separate StorageSettingsActivity
- Move signature setting to storage settings (relevant for WebDAV sync)
- Move data backup to separate BackupActivity with export/import
- Make "more" overflow button configurable in size
- Simplify SettingsActivity to 3 navigation buttons
- Add logbook rename/delete functionality
- Improve S/M/L button contrast with visible borders
