Introduce model error handling

[bohdanprog]

Fix #281.

Problem:
Previously, useBuffer silently swallowed load errors (only console.error), and useModel had no way to surface them to the caller. This made it impossible to show error UI or recover gracefully when a model fails to load.

Changes:
Add UseBufferResult interface to useBuffer returning { buffer, error } instead of FilamentBuffer | undefined
Add error state to FilamentModel type
Propagate buffer load errors through useModel as { state: 'error', error }
Update all useBuffer call sites to destructure the new return shape

Testing:
Run the new ModelErrorHandling example in examples/Shared/src/ModelErrorHandling.tsx. It intentionally loads from an invalid URL — you should see the error message rendered instead of a blank screen.

[bohdanprog]

Hey, @hannojg @mrousavy, could you take a look at my PR? It's JS-only changes that add error state propagation to useModel — so callers can now handle load failures gracefully instead of silently swallowing them.

Thanks