PM-5878: Add Show Dashboard flag to Marathon Match advanced settings

[jmgasper]

What was broken

The Work app in platform-ui had no control for the show_data_dashboard challenge metadata flag, so copilots could not enable the data dashboard graph on a Marathon Match challenge from the challenge editor. As noted in Slack, MM 166 launched without the dashboard because the flag had to be set manually against challenge-api-v6.

Root cause

The flag was only ever consumed, never authored. community-app reads the show_data_dashboard metadata entry (challenge-detail/Header/TabSelector) to decide whether to render the challenge dashboard tab, and challenge-api-v6 stores it as a generic key/value ChallengeMetadata row, but no field in the platform-ui challenge editor ever wrote it.

What was changed

• Added ShowDashboardField, a Show Dashboard checkbox that reads and writes the exact string-valued show_data_dashboard challenge metadata entry, following the existing StockArtsField / RegisteredMemberDownloadField metadata patterns.
• Rendered the checkbox in the challenge editor's Advanced Options section only for Marathon Match challenge types.
• Defaulted the checkbox to checked for fun challenges that have no saved show_data_dashboard value yet, and persisted that implied value so a save keeps the dashboard enabled. A saved value always wins, so the dashboard can be turned back off.
• Seeded show_data_dashboard during Marathon Match creation as true for fun challenges and false otherwise, so the default takes effect as soon as the challenge is set up.
• Documented the new field in the ChallengeEditorPage README.

No changes were needed in challenge-api-v6 or community-app: challenge metadata is a generic name/value collection, and community-app already renders the dashboard tab from this flag.

Added/updated tests

• New ShowDashboardField.spec.tsx covering the standard Marathon Match default (unchecked, no metadata written), the fun-challenge default (checked and persisted as "true"), saved metadata winning over the fun-challenge default, and toggling persisting exact string booleans.
• New parameterized ChallengeEditorForm.spec.tsx case asserting that creating a Marathon Match sends show_data_dashboard "true" for fun challenges and "false" otherwise. The FunChallengeField mock now binds to the form so the fun flag can be toggled in tests.

Validation

• npx eslint --quiet -c ./src/.eslintrc.js 'src/apps/work/src/pages/challenges/ChallengeEditorPage/**/*.{ts,tsx}' - clean
• npx tsc --noEmit -p tsconfig.json - clean
• yarn build:dev - succeeds
• CI=true npx craco test --watchAll=false --testPathPattern="src/apps/work" - 674 passed. The 13 remaining failures (AiReviewTab, ChallengePrizesField, PaymentFormModal, ReviewersField) are identical on a clean origin/dev checkout and unrelated to this change.

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