Add branch name to Mass Edit dialog

[thesobercoder]

The Mass Edit dialog lets you bulk update model, thinking level, planning mode, priority, and skip tests - but it doesn't include branch name.

When setting up a new project with many features, I want to assign them all to a feature branch without editing each one individually.

Steps:

1. Enter selection mode in board view
2. Select multiple features
3. Click Edit
4. Notice there's no option to change branch name

Expected: Add a branch name selector to the Mass Edit dialog (similar to the one in Add/Edit Feature dialogs)