fix(widgets): compact time offset popup layout

[LimiNode]

Summary

• keep timezone labels above their combo controls;
• keep offset format text above the manual input instead of extending the input row;
• avoid localized labels widening the popup through same-line composition;
• add popup_item_spacing and popup_window_padding for popup-local layout control;
• close ImGuiFramedWindow child-window style scopes immediately after BeginChild();
• restore the early return in classic layout when side_panel_width <= 0;
• add a regression CTest proving the classic no-side-panel path renders the main region once and never enters the side-panel path;
• document the window/child style-scope rule in the ImGuiX layout playbook;
• keep saved JSON configuration output at four-space indentation.

Verification

• cmake --build external/ImGuiX/build-mingw --parallel 36
• ctest --test-dir external/ImGuiX/build-mingw --output-on-failure (19/19)
• integrated mgc-gui build against the updated widget
• MGC CTest (17/17)
• git diff --check

This PR is intentionally separate from the already merged content-surface work.