[MPDX-9276] Fix cap comparisons

[canac]

Description

The individual and combined caps are confusing, and I had gotten some of them mixed up. This PR should fix all of the remaining issues. I also moved some of the logic to the server (like splitCapRequired) so that the logic is standardized between the frontend and server.

Depends on https://github.com/CruGlobal/mpdx_api/pull/3166

MPDX-9276

Testing

• Test case 1
  • Go to step 2 of a calculation in the test account
  • Check that the "Maximum Allowable Salary (CAP)" card says "You have a Board-approved Maximum Allowable Salary (CAP). Any adjustment that may exceed this cap must be submitted for further Board approval." and shows Joel's cap
• Test case 2
  • Impersonate caleb.cox@cru.org (my user is linked to someone without an exception cap in HCM)
  • Go to step 2 of a calculation
  • Set the location to somewhere expensive like NYC
  • Check that the "Maximum Allowable Salary (CAP)" card has the split checkbox
  • Change the location to None
  • Check that the "Maximum Allowable Salary (CAP)" card does not have the split checkbox

Checklist:

• I have given my PR a title with the format "MPDX-(JIRA#) (summary sentence max 80 chars)"
• I have applied the appropriate labels (Add the label "Preview" to automatically create a preview environment)
• I have run the Claude Code /pr-review command locally and fixed any relevant suggestions
• I have requested a review from another person on the project
• I have tested my changes in preview or in staging
• I have cleaned up my commit history

[github-actions]

Preview branch generated at https://9276-fix-cap-comparisons.d3dytjb8adxkk5.amplifyapp.com

[github-actions]

Bundle sizes [mpdx-react]

Compared against 5939d0e

No significant changes found