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
-->
## 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 -->

---------

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