Isolate Qt and GLFW ImGui frontends

[mtao]

Summary

• gate Qt and GLFW/ImGui sources, tools, and examples by their frontend options
• reject ImGui builds without the currently required GLFW platform backend
• constrain the bundled ImGui fallback to Vulkan + GLFW only
• add CI configuration/compile checks for Qt-only and GLFW+ImGui matrices

Verified configurations

• -Dqt=true -Dglfw=false -Dimgui=false
• -Dqt=false -Dglfw=true -Dimgui=true
• -Dqt=true -Dglfw=false -Dimgui=true fails with a clear configuration error

Using a clean current quiver checkout, full builds complete for:

• mesh_viewer_qt
• mesh_viewer_glfw
• image_viewer_glfw

All three executables pass --help smoke tests. The earlier spatial API errors were caused by a stale local quiver subproject cache; balsa source already uses the current scalar-first AABB/KDOP/BVH API.

Stack

This PR targets feature/embed-shader-resources and should be rebased onto main after #14 merges.