Allow highlighting a group of nodes using a colored rectangle (Area Highlight)

### Problem

When working with large mind maps, it becomes difficult to visually identify related groups of nodes.

For example, in a family tree or a project map, I may want to visually group several nodes together without changing their individual colors or the tree structure.

Currently, there is no way to highlight an arbitrary region of the map.

### Proposed Solution

Introduce an **Area Highlight** feature.

The user should be able to draw a colored rectangle (or any simple highlight region) around a group of nodes.

The highlight should:

* Stay behind the nodes.
* Be resizable.
* Be movable.
* Support different background colors.
* Optionally allow transparency.

### Benefits

* Improves readability of large maps.
* Makes presentations much clearer.
* Similar to using colored areas in diagramming tools such as draw.io or Miro.

The issue names no files or tests. Start by locating the mind-map canvas, node rendering, and interaction entry points, then determine how movable and resizable elements are represented. Done means users can create, move, resize, reorder behind nodes, and recolor a simple highlight region, with transparency if supported.

Written by the indexing model from the issue text.