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/scene/text/ttf_glyph.rs

@@ -205,6 +205,7 @@ fn shape_cache() -> &'static Mutex<ShapeCache> {
 
 /// Pixel font size the layout runs at. Positions and outlines are normalized
 /// out of pixel space afterwards, so the exact value only affects rounding.
+#[allow(dead_code)]
 const SHAPE_FS: f32 = 1000.0;
 
 /// Shared cosmic-text layout engine, loaded once with the system fonts. Held