PM-5761: Count every design submission concept

[jmgasper]

What was broken

Challenge detail and listing responses counted multiple Design concepts from one member as a single submission. Community App renders those API counters directly, so five concepts could appear as one submission.

Root cause

Challenge API recomputed both numOfSubmissions and numOfCheckpointSubmissions with memberId as the distinct identity for every track. That is correct for replacement attempts on non-Design challenges, but Design uploads can represent separate concepts.

What was changed

• Use submission ID as the distinct counter identity for Design challenges.
• Preserve member-based counting for all other tracks.
• Apply the Design rule to both checkpoint and non-checkpoint submissions.

Any added/updated tests

• Added a regression test with five Design concepts from one member, covering both final and checkpoint counters in challenge detail and listing responses.
• Preserved and reran the existing Development regression to verify repeated attempts still count once per member.
• pnpm test -- --grep 'returns latest-member submission counters|counts every Design submission', pnpm lint, and pnpm build pass.

The full pnpm test command was also run. It reaches and passes the PM-5761 tests, but the current develop baseline has unrelated failures across legacy validation assertions and test database setup.