This PR implements [feature request] #14845
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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.
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Co-authored-by: DarkSky <25152247+darkskygit@users.noreply.github.com>