Add banner UI for chunk load errors
Instead of showing a full-page "This page crashed" error for ChunkLoadError,
display a top-fixed banner while preserving the visible content as a frozen
snapshot:
- Banner shows "This page couldn't be fully loaded" headline
- Contextual detail: "A required file couldn't be loaded." (default) or
"You're offline." (when offline)
- Single CTA: "Reload page" button
- Preserves frozen HTML snapshot of page content underneath
- Neutral styling (no red, no modal, no dismiss)
- Accessible: role="alert", aria-live="assertive"
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
