Command palette: async sources hang when provider throws

When an async source passed to the command palette throws (network error, bad JSON), the loading spinner never resolves and the whole palette becomes unresponsive — even for local commands.

Unhandled Promise Rejection: TypeError: results is not iterable
   at CommandPalette.mergeSources (command-palette.tsx:141)
   at async Promise.all (index 2)

Fix direction: isolate each source with Promise.allSettled, render per-source error rows, and keep local commands interactive while remote sources are pending.