chore: silence native and wasm build warnings

Gate target-specific code so neither build warns about the other's items:
native-only helpers (view, window icon, file dialogs, file association, plugin
host/guard/registry, self-update) are cfg'd to non-wasm; the wasm-only font
fallback (script_of / cjk_lang / request) is cfg'd to wasm. Drop an unused mut,
allow the few genuinely-conditional dead items (mesh_click_hit, GPU layout
helpers, parity stubs), and gate the embedded PlotSettings-unrelated dead
constants. Both targets now build clean (only the unavoidable nom/quick-xml
future-incompat dependency notice remains).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/pipeline/hatch_batched_gpu.rs

@@ -106,6 +106,7 @@ pub struct HatchBatchedVertex {
 }
 
 impl HatchBatchedVertex {
+    #[allow(dead_code)]
     pub fn layout<'a>() -> wgpu::VertexBufferLayout<'a> {
         wgpu::VertexBufferLayout {
             array_stride: std::mem::size_of::<HatchBatchedVertex>() as u64,
@@ -450,6 +451,7 @@ impl HatchBatchedGpu {
     /// constructed once at startup. All four bindings are read-only
     /// storage and visible to both VS (AABB+visibility lookup) and FS
     /// (boundary / family / dash sampling).
+    #[allow(dead_code)]
     pub fn bind_group_layout(device: &wgpu::Device) -> wgpu::BindGroupLayout {
         let entry = |binding: u32| wgpu::BindGroupLayoutEntry {
             binding,