fix(webgpu): drop the removed UICustomBinding::_pad from the CustomShader example

Examples
- CustomShader did not build for wasm32-wasip1: its DOM branch still set
  UICustomBinding::_pad, which 097cc37 (#27) turned into format. The
  invalid designator left the bindings array of incomplete type, so the
  Load call after it found no overload either. The designator is dropped
  rather than renamed, as HDRBloom does: format is 0 for anything but a
  StorageTexture binding, and a buffer binding has no texel format to
  give. The native branch was never affected.
- Every other example with a DOM path (HelloUI, HelloDom, HDRBloom,
  RayQueryPick, RTMultiShadow, RTStress, RTVolume, VulkanTriangle,
  Sponza) builds for wasm at this commit. Nothing builds them
  otherwise, which is how this one fell behind.

Closes #158.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/CustomShader/main.cpp

@@ -64,7 +64,6 @@ int main() {
         { .group   = 2,
           .binding = 0,
           .kind    = UICustomBindingKind::Buffer,
-          ._pad    = 0,
           .pushOffset = static_cast<std::uint32_t>(offsetof(Crafter::UIDispatchHeader, itemBuffer)) },
     };
     inverseCircle.Load(std::filesystem::path("inverse-circle.comp.wgsl"), invBindings);