fix: avoid showing GitHub app install modal while repos are loading

[Che-Zhu]

Summary

• add an explicit loading step to the Import GitHub dialog
• show Import from GitHub with loading state while checking installation and fetching repos
• only show Install GitHub App when installation is actually missing

Why

Users with existing GitHub installation were briefly shown the install modal before repos loaded, which looked like a bug.

Validation

• Type check: pnpm exec tsc --noEmit --pretty false
• Manual flow expectation: opening Import now shows loading first, then repo list when installation exists

[github-actions]

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  Commit: 0822db2ee4a845629be220854cc71c4cd5fdc531
  Branch: codex/projects-import-button-investigation

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