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 <[email protected]>

src/scene/mod.rs

@@ -2429,6 +2429,7 @@ impl Scene {
 
     /// Pick a meshed 3D solid by clicking on its shaded body (face), not just
     /// its thin projected edges. Returns the front-most mesh under `cursor`.
+    #[allow(dead_code)]
     pub fn mesh_click_hit(
         &self,
         cursor: iced::Point,