feat: add loading skeleton distinct from not-found state to stream details page (Closes #1088) - #1155

Closes #1088

Summary

• Replace the minimal spinner loading state with a full-page skeleton UI that mimics the stream details layout: header, overview, stats cards, progress bar, action buttons, and event history
• Each skeleton element uses animate-pulse shimmer placeholders styled to match the dark theme
• The not-found/error state remains visually distinct with a red AlertTriangle icon — no risk of confusing the two states
• Loading region has proper role="status" and aria-label for accessibility

Testing

• Added 4 unit tests covering: skeleton renders while fetch is in-flight, transition from skeleton to content on success, transition from skeleton to not-found on 404, and error message display on API failure