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/app/update.rs

@@ -238,6 +238,7 @@ impl OpenCADStudio {
 
     /// Snapshot of disabled plugin ids — lets the registry skip them while it
     /// holds a `&mut` borrow of the app via `HostSession`.
+    #[cfg(not(target_arch = "wasm32"))]
     pub(crate) fn disabled_plugin_ids(&self) -> rustc_hash::FxHashSet<String> {
         self.disabled_plugins.clone()
     }