Summary

• Extract the prompt detail page platform grouping helper into a local module, mirroring the accepted insights grouping pattern from fix(web): group insights results by platform #235.
• Group prompt detail results by platform only, so provider model slug changes do not split one logical platform into duplicate cards.
• Keep the group model/region metadata sourced from the latest run and add Vitest coverage for slug drift plus distinct-platform separation.

Related issue

Closes #236

Why

The prompt detail page had its own groupByPlatform() copy that keyed groups by platform|modelUsed. When a provider slug drifted over time, the same platform could appear as two cards even though it was the same logical platform history.

Validation

• corepack yarn install --frozen-lockfile
• corepack yarn test 'src/app/[locale]/(dashboard)/dashboard/prompts/[id]/grouping.test.ts'
• corepack yarn prettier --check 'src/app/[locale]/(dashboard)/dashboard/prompts/[id]/page.tsx' 'src/app/[locale]/(dashboard)/dashboard/prompts/[id]/grouping.ts' 'src/app/[locale]/(dashboard)/dashboard/prompts/[id]/grouping.test.ts'
• corepack yarn eslint 'src/app/[locale]/(dashboard)/dashboard/prompts/[id]/page.tsx' 'src/app/[locale]/(dashboard)/dashboard/prompts/[id]/grouping.ts' 'src/app/[locale]/(dashboard)/dashboard/prompts/[id]/grouping.test.ts'
• corepack yarn test
• corepack yarn typecheck
• corepack yarn format:check
• corepack yarn lint (0 errors; existing unrelated warnings remain in other files)
• git diff --cached --check
• git diff --check HEAD~1..HEAD

Scope

Display/grouping layer only. No data fetching, RPC, or stored data changes.