Add sidebar worktree reload button with shared spinner state

[ivankristianto]

Summary

• add isRefreshingWorktrees state in RepositoriesFeature to track reload progress
• wire refresh state to the existing .refreshWorktrees flow so keyboard shortcut, periodic refresh, and button share the same loading state
• add a reload button in the sidebar footer with a spinning arrow.clockwise icon while refreshing
• place the reload button between Help and Archived Worktrees in the footer

Verification

• xcodebuild test -project supacode.xcodeproj -scheme supacode -destination "platform=macOS" -only-testing:supacodeTests/RepositoriesFeatureTests CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY="" -skipMacroValidation
• make build-app