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 <[email protected]>

src/io/file_association.rs

@@ -37,6 +37,7 @@ const APP_ID: &str = "io.github.HakanSeven12.OpenCadStudio";
 /// Idempotent and best-effort; safe to call on every launch. Runs synchronously
 /// (registry / small file writes), so callers should invoke it off the UI
 /// thread.
+#[cfg(not(target_arch = "wasm32"))]
 pub fn register_as_handler() -> Result<(), String> {
     #[cfg(target_os = "windows")]
     {

src/io/mod.rs

@@ -31,6 +31,7 @@ const PHASE_FINALIZING: u8 = 3;
 /// Show the file picker and return the chosen path plus its size in bytes.
 /// Returning size up-front lets the loading overlay display "47.3 MB" before
 /// the parser thread starts.
+#[cfg(not(target_arch = "wasm32"))]
 pub async fn pick_open_path() -> Option<(PathBuf, u64)> {
     let handle = rfd::AsyncFileDialog::new()
         .set_title("Open CAD file")