fix: show loading state in git sync button while repo data is being fetched [INS-2610]

[pavkout]

Summary

• When opening a git-connected project, the git sync button was immediately showing "Git is not connected" because isSynced evaluated to false while gitRepoDataFetcher.data was still undefined (the fetcher is triggered lazily via a useEffect after mount).
• Added isLoadingGitRepo — true when a gitRepository.uri exists but fetcher data has not yet arrived — to distinguish the loading phase from a genuinely disconnected state.
• The sync button now renders a spinner + "Loading..." label during that window, replacing the misleading "Git is not connected" flash.

[Copilot]

Pull request overview

This PR improves the Git project sync dropdown UX by preventing a brief, misleading “Git is not connected” state while the repository metadata is still being lazily fetched after mount.

Changes:

• Adds an isLoadingGitRepo state derived from gitRepository.uri + absence of gitRepoDataFetcher.data.
• Renders a spinner + “Connecting...” placeholder row during the initial repo-data fetch window.

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