[Refactor] Use uniq helper in app-management-client

[gonzaloriestra]

WHY are these changes introduced?

Follows the project's convention of using the uniq helper from @shopify/cli-kit for array deduplication to improve code readability and maintainability.

WHAT is this pull request doing?

Refactors allowedTemplates in packages/app/src/cli/utilities/developer-platform-client/app-management-client.ts to use:

• The uniq helper from @shopify/cli-kit/common/array instead of Array.from(new Set(...)).
• .flatMap() instead of .map(...).flat().

How to test your changes?

CI

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

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PR created automatically by Jules for task 14663225430183466267 started by @gonzaloriestra

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