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/plugin/registry.rs

@@ -15,6 +15,7 @@ pub fn all_ribbon_modules() -> Vec<Box<dyn CadModule>> {
 pub fn ribbon_modules_enabled(
     disabled: &rustc_hash::FxHashSet<String>,
 ) -> Vec<Box<dyn CadModule>> {
+    #[cfg_attr(target_arch = "wasm32", allow(unused_mut))]
     let mut core = core_registry::all_modules();
     // Dynamically-loaded external plugins contribute tabs (their libraries stay
     // resident for the session, so these vtables remain valid).