[codex] Align challenge prizes and billing layout

[jmgasper]

What changed

• aligned the challenge-prize type selector with the fixed-width prize input so the USD and Points radios sit over the right side of the field
• changed the selected challenge-prize radio accent from blue to the challenge editor's green state
• fixed the desktop Prizes & Billing layout to keep the Challenge Prizes and Copilot Fee inputs at fixed widths and leave whitespace to the right instead of stretching across the row
• updated the local Challenge Editor README to document the fixed-width layout and right-aligned prize selector behavior

Why it changed

The current desktop layout stretched the prizes and copilot inputs across the available row width, which left the prize-type radios visually detached from the prize field and kept the selected radio in the wrong blue accent. The screenshot target shows fixed-width fields with the selector aligned over the field area and using the editor's green selected state.

Impact

The work-app challenge edit and view pages now present the Prizes & Billing row with the intended spacing and selector styling while preserving the existing responsive stack on smaller screens.

Validation

• source "$NVM_DIR/nvm.sh" && nvm use && yarn lint
• source "$NVM_DIR/nvm.sh" && nvm use && yarn run build (succeeds with existing repo-wide build warnings about CSS order and non-blocking lint-style warnings emitted during the production bundle)

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.