Addresses issue #567 where selecting "Custom interval..." from the
backup polling dropdown would revert to a preset option if the current
custom minutes value happened to match a predefined interval.
The bug occurred because:
1. User selects "Custom interval..." from dropdown
2. Code sets interval based on current customMinutes value
3. If that value matches a preset (e.g., 60 min = 1 hour), the
computed select value returns the preset instead of 'custom'
4. Dropdown reverts, hiding the custom input field
Fix introduces a dedicated state variable (backupPollingUseCustom)
to explicitly track whether custom mode is active, independent of
whether the current interval value matches a preset option.
Changes:
- Add backupPollingUseCustom signal to track custom mode state
- Update backupIntervalSelectValue() to check custom flag first
- Set/clear custom flag in dropdown onChange handler
- Initialize custom flag when loading settings from API
Related to #567
fd4182563e