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/app/helpers.rs

@@ -286,6 +286,7 @@ pub(super) fn title_case_word(value: &str) -> String {
 // ── Window icon ────────────────────────────────────────────────────────────
 
 /// Builds a 32×32 RGBA icon: red background with OCS drawn in white pixels.
+#[cfg(not(target_arch = "wasm32"))]
 pub(super) fn build_window_icon() -> Vec<u8> {
     const W: usize = 32;
     const SZ: usize = W * W * 4;