Make data rollover optional in Smart Data Allocation

[lfos]

Checklist

• I made sure that there are no similar feature requests - open or closed.
• I have taken the time to fill in all the required details in an appropriate descriptive way. I understand that the feature request will be dismissed otherwise.
• This issue contains only one feature request.

Describe the Feature you'd like

Add a slider to the Smart Data Allocation settings to toggle data rollovers. If enabled, the behavior matches the current behavior. If disabled, at the end of a day, remaining data is always spread evenly across remaining days.

Why do you want this feature?

Avoid accumulating data in a single big chunk. Smart Data Allocation is a convenient way to avoid unplanned data spikes in limited-data settings; rollovers somewhat limit the utility of Smart Data Allocation for this use case. Disabled rollovers also make Smart Data Allocation deterministic (i.e., tiny updates to the plan do not reset state).

Additional context

No response