[Due for payment 2026-06-22] [DB MileageRate] App: Update confirmation and rate selection UX for date-aware mileage rates

Use date-aware rate auto-fill on confirmation, show auto-updated tooltip when date changes alter the rate, and display rate date range text in the rate picker.

https://docs.google.com/document/d/1ByolNK4XOhNksdl_KDq7TcwJgeXgxDzW384H7SIjMU0/edit?pli=1&tab=t.0#bookmark=id.ty5laufd83y9

<details><summary>Issue Owner</summary>Current Issue Owner: @Krishna2323</details>