Prooftree-- natural deduction trees

[FallenSky2077]

This PR adds proof-tree support to Mogan using a macro-based approach with environment variables.

Features

• Three proof-tree variants: unlabeled, right-labeled, left-labeled
• Keyboard shortcuts: Insert proof-trees from math mode
• Toolbar support: Focus menu with variant dropdown, insert/remove buttons
• Structured navigation: Arrow keys navigate between conclusion, premises, and labels
• Structured editing: Insert/remove premises, insert sub-proofs above/below

Implementation Approach

Instead of adding new C++ primitives, proof-trees are implemented as:
(with "tree-mode" "proof" "tree-label-pos" "" (tree conclusion premise1 premise2 ...))

• tree-mode = "proof" enables proof-tree rendering
• tree-label-pos = "none", "left", or "right" controls label position

Delete/Backspace Behavior

• Visual order: left-label → premises → right-label
• Boundary delete moves to adjacent field
• Empty premise deletion (if multiple exist)
• All-empty removes entire proof-tree