fix(app): build the fontconfig face helper only where it is used

Windows and macOS find their CJK font by path, so on those targets the
helper was dead code and the Windows cross gate refused it.

crates/pstr-app/src/theme.rs

@@ -277,6 +277,7 @@ fn cjk_font_data(bytes: Vec<u8>, index: u32) -> Option<egui::FontData> {
 /// Fedora's variable Noto Sans CJK answers 262144, instance 4 of face 0. The
 /// instance is not a face, and passing it to egui as one stopped the app at
 /// start.
+#[cfg(any(target_os = "linux", test))]
 fn fontconfig_face(index: u32) -> u32 {
     index & 0xFFFF
 }