HFI Calculator: Configure dynamic prep date range windows

**As a** *HFI User (plans/co)* **I want** *to configure start days and end days to automatically create prep periods* **So That** *I don't have to manually set prep period ranges*

**Additional Context**
- Feedback from DK and SI in NWFC
- Screen shot of meeting occurrence creation in Outlook for inspo:

**Acceptance Criteria**
- [ ] Given I submit a date range configuration, When the current prep period finishes, Then the next date range creates a prep period based on the date range

**Definition of Done**
- Ready to Demo in Sprint Review
- Does what I have made have appropriate test coverage?
- Documentation and/or scientific documentation exists and can be found
- Peer Reviewed by 2 people on the team
- Manual testing of all PRs in Dev and Prod
- Merged

No files, tests, or entry points are named. Start by locating the HFI Calculator code that creates prep periods and determine where date-range configuration and current-period completion are handled; done means the next period is created from the submitted range, with appropriate test coverage and documentation.

Written by the indexing model from the issue text.