PM-5755: Simplify design screener assignment

[jmgasper]

What was broken

Copilot-only users editing Design Challenges saw the complete review configuration and had to assign checkpoint and final screeners independently.

Root cause

The Review section did not distinguish copilot-only Design Challenge editing from admin and manager configuration, and each screening reviewer synchronized only its own resource role.

What was changed

Show a single Screener selector for copilot-only Design Challenges while leaving the full interface for administrators and managers. Synchronize the selected member across Checkpoint Screener and Screener resources, preserve hidden reviewer defaults, and guard replacement, clearing, legacy assignments, request failures, and overlapping updates.

Any added/updated tests

Added coverage for role and challenge-type gating, simplified rendering, two-round and one-round assignment, replacement and clearing, legacy handle-only resources, fetch failures, in-flight locking, and preservation of reviewer configuration.

Validation completed:

• yarn lint passed.
• yarn run build passed.
• Ticket-affected test runs passed 118 tests.
• The required yarn test:no-watch command passed 222 of 240 suites. The remaining 18 suites reproduce on the untouched origin/dev baseline and are unrelated to PM-5755.