HFI Calculator: Disable editing of previous prep periods

**As a** *Prep Planner* **I want** *to be able to view previous prep periods without being able to edit them* **So That** *I can easily understand that I've switched to a view-only prep period, where I'm only retrieving historical values*

**Additional Context**
- Editing only makes sense for current a future prep periods.

**Acceptance Criteria**
- [ ] Given I switch to a prep period where the date range is in the past, Then I am unable to edit fields regardless of permissions

**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. Locate the HFI Calculator's prep-period fields, date-range handling, and permission checks, then verify that a past period cannot be edited regardless of permissions and add appropriate test coverage.

Written by the indexing model from the issue text.