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/host.rs

@@ -4,4 +4,5 @@
 // The session adapter the registry wraps `app` in to dispatch to plugins. The
 // plugin-facing contract — `BuiltinPlugin` + `HostApi` — lives in
 // `ocs_plugin_api`; external cdylibs target it directly.
+#[cfg(not(target_arch = "wasm32"))]
 pub(crate) use crate::app::plugin_host::HostSession;