refactor: DRY gexport page layout wrapper

[somethingwithproof]

Problem

gexport.php repeats the same page layout wrapper sequence (top_header(), renderer, bottom_footer()) in action routing.

Proposed change

• add a shared UI layout wrapper helper for gexport pages
• route edit and default actions through the helper
• preserve behavior while reducing duplicated wrapper logic

Tests

• add standalone regression test for wrapper call ordering and callback execution
• include routing assertions that actions use the helper
• run syntax checks for touched files