refactor: DRY quicktree action form wrappers

[somethingwithproof]

Problem

quicktree.php repeats action-form wrapper scaffolding in multiple action branches (add_tree, add_branch), including conditional header display and identical form/box begin/end calls.

Proposed change

• add shared helpers for quicktree action-form wrapper begin/end
• use shared helpers in add-tree and add-branch paths
• preserve behavior while reducing duplicated wrapper logic

Tests

• add standalone regression tests for helper wrapper call order
• add source assertions for helper usage in both action branches
• run syntax checks for touched files