Add Loading Indicator for Schema Designer

[yoleichen]

When a user opens the schema designer, there is a noticeable few-second lag until the diagram is fully loaded. To improve the user experience, a spinning wheel or loading indicator should be added to indicate that the diagram is still loading.

(cc. @jdesai-24 , @croblesm)

Recording.2025-03-24.225039.mov

[aasimkhan30]

Added loading spinner