fix: add error boundaries to prevent full app crashes (#1270)

[abu-abdullah22]

Fixes #1270

Summary

Adds React Error Boundaries to prevent single-component errors from crashing
the entire embedded chat widget.

Changes

• New ErrorBoundary component (packages/react/src/views/ErrorBoundary/)
  with a fallback UI (retry + reload) using the app's existing Box, Button,
  and Icon components from @embeddedchat/ui-elements
• Wrapped the root EmbeddedChat component so any unhandled error shows the
  fallback instead of a blank screen
• Wrapped ChatBody and ChatInput separately inside ChatLayout so an
  error in one doesn't take down the other
• Added a changeset (patch, @embeddedchat/react)
• Added ErrorBoundary.test.js using @testing-library/react

Note for reviewers

packages/react currently has jest as a dependency but no working Jest
config/test script wired up — running npx jest fails with
"Cannot use import statement outside a module" since Babel isn't connected
to Jest's transform pipeline. I added the test file following existing
testing-library patterns used elsewhere in the codebase, but couldn't verify
it runs locally due to this pre-existing gap. Happy to add a jest.config.js

• babel-jest setup as a follow-up if you'd like it in scope here.

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