Date picker UI rework

Out of scope for this PR because it's about the picker state logic, but I find this interaction weird. I make the range invalid by setting end time before start time, which makes sense, but then the way we keep the invalid range around is kind of odd: if I click two different days, then we get a valid range because of course 4/3 10pm - 4/4 9pm is a valid range. But then I click the same day twice and it becomes invalid again because 4/3 10pm - 4/3 9pm is invalid. I understand the logic, and maybe it's just consistent and there isn't a better way, but I feel like maybe we could be more aggressive in resetting the times, or we could make clearer visually that the start time is associated with the start date, maybe by displaying the date above the times. Right now it feels a little disjointed because you have to look at both the input at the bottom of the popover and the thing at the top to really understand what you've chosen. The times at the bottom look like a time range within a single day, which is only true when the start and end days are the same.

Originally posted by @david-crespo in https://github.com/oxidecomputer/console/issues/2780#issuecomment-2779184014

https://github.com/user-attachments/assets/f3b74f6d-7c56-43a5-9d1a-47a9d8c0f813

No file or test is named. Start by locating the date picker UI and reviewing the interaction described in issue 2780, including the attached example; agree on the intended invalid-range reset behavior and how dates should be associated with times before defining completion criteria.