Build: the wasm check is warning-free; one file-dialog check instead of two

- ui-egui: the font-install poll (#2576) only runs on native builds, but its
  timestamp field, its initializer and FONT_INSTALL_POLL were compiled for
  wasm too, where nothing read them. They're gated like the code that reads
  them.
- engine: `Session::store_profile` is only called by the native Load
  Profile (cmd/colormgmt.rs); gated the same way.
- app (macOS): `App::logic_frame` checked `!file_dialog_open()` twice, one
  `if` nested in an identical one, with the comment repeated. One check.

`cargo xtask wasm` runs `cargo check` without `-D warnings`, so these
warnings didn't fail CI; after this change it prints none.

crates/engine/src/store.rs

@@ -204,6 +204,7 @@ impl Session {
     }
 
     /// Write the profile `name` (its file's `bytes`) to the store; `false` when it didn't go there.
+    #[cfg(not(target_arch = "wasm32"))]
     pub(crate) fn store_profile(&self, name: &str, bytes: &[u8]) -> bool {
         self.profile_dir.as_deref().is_some_and(|dir| put(dir, &format!("{}.{PROFILE_EXT}", file_stem(name)), bytes))
     }