Fix recurring Linux device, assembly, and shutdown crashes

[Jubliano-sama]

Description

Fix three independent crash paths observed in the Linux v0.4.0 AppImage:

1. Backport OrcaSlicer's targeted WebKitGTK workaround for the hidden vue-resize <object> probe used by Mainsail < 2.16.1 and Fluidd < 1.37.0. The workaround is injected at document start and restored after RemoveAllUserScripts(). This is the reviewed fix from Fix Linux segfault when loading Fluidd/Mainsail printer hosts (#7210) OrcaSlicer/OrcaSlicer#13724 for Segfaults on Linux after connected to Mainsail/fluidd OrcaSlicer/OrcaSlicer#7210.
2. Guard the assembly controls until both m_assemble_view_data and its model_objects_clipper() are available. The asynchronous reset callback performs the same lifetime check before dereferencing the clipper.
3. Ignore late wxGTK EDITING_DONE events after application shutdown begins, before the callback can mark an already-reset plater canvas dirty.

The changes do not modify Wave Overhangs slicing behavior, presets, translations, or profile formats.

Crash evidence

• Two dumps entered WebKit/JSC from the embedded Mainsail 2.14.0 device page and matched the vue-resize crash addressed upstream.
• One dump faulted in GLCanvas3D::_render_assemble_control() after model_objects_clipper() returned null.
• One dump faulted through ObjectList::OnEditingDone() -> Plater::set_current_canvas_as_dirty() while the application was closing.

Screenshots/Recordings/Graphs

Not applicable: these are lifetime and embedded-WebView crash fixes with no intended visual change.

Tests

• git diff --check origin/main...HEAD
• ./build_linux.sh -sir -j 6
  • Full release build completed (646/646).
  • OrcaSlicer_profile_validator built successfully.
  • AppImage dependency audit passed.
• Generated OrcaSlicerWaveOverhangs_Linux_V2.4.0-dev.AppImage successfully.
  • SHA-256: b6dd59cea78081ca1444818fd00850e3adfcc9024f27679d62139cbbac175981
• Launched the packaged AppImage on KDE Plasma/Wayland, restored the existing project, and closed it through the normal window-close path.
• Confirmed the smoke test produced no new core dump.

No focused unit test currently covers wxGTK teardown event ordering or the embedded WebKit document lifecycle.