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/external.rs

@@ -79,6 +79,7 @@ pub fn plugins_dir() -> Option<PathBuf> {
 
 /// Delete an installed package's folder. It stays loaded for the current
 /// session (the library is resident); the removal takes effect on next start.
+#[cfg(not(target_arch = "wasm32"))]
 pub fn uninstall(id: &str) -> Result<(), String> {
     let dir = plugins_dir().ok_or("cannot locate the plugins folder")?.join(id);
     if dir.is_dir() {