feat(frontend): add skeleton loading state to StellarChatInterface (#1284)

## Summary
- add an announced loading skeleton state to StellarChatInterface
- add regression coverage for the new loading behavior

## Validation
- pnpm vitest run src/components/__tests__/StellarChatInterface.skeleton.test.tsx
- pnpm typecheck
- pnpm lint

Closes #1193

Author: bamiebot-maker