[MPDX-9200] Add additional info box

[canac]

Description

Implement the additional info textfield for users to provide more context when their request exceeds the limits.

Depends on https://github.com/CruGlobal/mpdx_api/pull/3134

MPDX-9200

Testing

• Go to a salary request
• Go to step 3
• Enter a large value for the requested salary like $100K
• Check that the additional information box shows up and that text typed there is persisted

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://9200-additional-info.d3dytjb8adxkk5.amplifyapp.com

[canac]

This changed intentionally. Before this message shows if one user is over their individual cap, but that doesn't require approval. Approvals are required if the combined requested amount is above the combined cap.