Add beam join command

[gitgrahamdunn]

Summary

• adds a Join beams frame command for the roadmap Beam joins item
• trims or extends selected beams so their nearest centerline endpoints meet at a shared centerline intersection
• supports two or more selected beams; the first two selected beams define the join point, and additional selected beams join to that same point
• adds the command to the Frame tools toolbar/menu and marks the roadmap item complete

Behavior

The command preserves each beam's axis and section rotation. It only updates the selected beam's base point and/or Height, choosing the beam end closest to the join point.

Validation

• python3 -m py_compile fCmd.py CFrame.py InitGui.py Quetzal_tooltips.py
• git diff --check
• FreeCAD command-mode test with two perpendicular test beams: both joined at (50, 0, 0) and adjusted to 50 mm length