feat(editor): add block button for hovering blocks (#14879)

This PR implements [feature request] #14845 

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **New Features**
* Add-block control that appears when hovering blocks in page mode to
insert and auto-focus a new paragraph; control hides after insertion.

* **Improvements**
* Improved hover and interaction handling to avoid accidental triggers
when interacting with the drag handle or add-block control.
* Consistent sizing, positioning, and visibility behavior for the
add-block control.

* **Style**
  * Moved heading icon slightly for improved visual alignment.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: DarkSky <25152247+darkskygit@users.noreply.github.com>

yarn.lock

@@ -2932,6 +2932,7 @@ __metadata:
     "@blocksuite/affine-components": "workspace:*"
     "@blocksuite/affine-ext-loader": "workspace:*"
     "@blocksuite/affine-model": "workspace:*"
+    "@blocksuite/affine-rich-text": "workspace:*"
     "@blocksuite/affine-shared": "workspace:*"
     "@blocksuite/global": "workspace:*"
     "@blocksuite/icons": "npm:^2.2.17"