### What
Adds support for rendering an optional image above the document title
using document metadata.
### Why
Provides a visual identifier for documents and improves readability for
users who rely on visual cues.
### How
- Reads `headerImage` from document metadata (if present)
- Renders the image above the editor when present
- Fully optional and non-breaking
- No BlockSuite or data model changes
### Related
fix#14240
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Documents can now display header images in the page editor. When a
header image is available, it appears above the editor content,
enhancing visual presentation and providing better context for your
documents.
<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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