Fix trajectory fetch error handling and add loading/error UI

[meganrm]

Problem

Estimated review size: medium

fetch3DTrajectory's catch block only did console.log(error) and never reported failure to the caller, so App.tsx unconditionally set trajectoryStatus to LOADED even after a failed fetch. Separately, nothing in the app ever rendered UI based on trajectoryStatus at all — a loading or failed fetch just left the viewer blank with no feedback.

Fixes #223

Solution

fetch3DTrajectory now returns a success boolean instead of swallowing errors. App.tsx sets TrajectoryStatus.ERROR on failure instead of always LOADED. trajectoryStatus is now exposed through SimulariumSimulationContext so Viewer.tsx can consume it, and shows a loading spinner while fetching and an error message + retry button on failure (retry resets status to INITIAL, re-entering the existing fetch effect).

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Change summary:

• fetch3DTrajectory returns a success boolean instead of swallowing errors
• App.tsx sets TrajectoryStatus.ERROR on failure instead of always LOADED
• trajectoryStatus/setTrajectoryStatus exposed through SimulariumSimulationContext
• Viewer.tsx shows a loading spinner while fetching and an error message + retry button on failure

Steps to Verify:

1. Navigate to a page with a precomputed trajectory
2. Confirm a loading spinner appears while it fetches
3. Temporarily point a trajectory URL at a broken/404 URL
4. Confirm an error message + retry button appear instead of a blank screen
5. Click retry — confirm it re-attempts the fetch

Screenshots (optional):

Keyfiles (delete if not relevant):

1. src/utils/fetch3DTrajectory.ts
2. src/App.tsx
3. src/components/Viewer.tsx
4. src/simulation/context.tsx

Thanks for contributing!