Adds a new "Every 6 hours" frequency option to the periodic backup settings.
To maintain consistency with the existing preference values, which are stored in days, this new option is represented internally as a fractional value of `0.25` days.
The implementation includes:
- Adding the new string resource and updating the preference arrays.
- Changing the preference type in `AppSettings.kt` from `Long` to `Float` to accommodate the fractional value.
- Updating the millisecond conversion logic to correctly calculate the interval from a float value in days.
This approach avoids a complex data migration and is simpler and safer than changing the base unit for all values from days to hours.
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