Offload styles from main application stylesheet

[aduth]

🛠 Summary of changes

Removes several items from the main application stylesheet to reduce its size:

1. Remove unused usa-step-indicator styles
   • IdV step indicator is fully custom and does not use design system styles
2. Split account layout navigation styles to separate stylesheet
3. Move usa-collection styles to document capture as the only place it's used

Performance Impact:

NODE_ENV=production yarn build:css && brotli-size app/assets/builds/application.css

Before: 22.7 kB
After: 21.6 kB
Diff: -1.1 kB (-4.8%)

📜 Testing Plan

Verify no regressions in affected styles, particularly account page header and navigation, and address search functionality for in-person proofing during document capture.