feat(export): surface typst warnings and glyph-coverage notice

Typst compile warnings were discarded. Capture them via the Warned
result and emit a pdf-warnings event (only from the export path, not
the live preview) that App shows as a non-fatal toast; the export still
succeeds. Also scan book text before reporting success and warn when it
contains scripts the embedded Latin fonts cannot render (CJK, Arabic,
Hebrew, Devanagari, Thai, emoji).

Claude-Session: https://claude.ai/code/session_01RzTWrLiy4zGuw2hZSQGamk

src/App.tsx

@@ -23,8 +23,12 @@ function App() {
       else if (event.payload === "export-epub") runExport("epub");
       else if (event.payload === "check-updates") checkForUpdates(false);
     });
+    const unlistenWarn = listen<string>("pdf-warnings", (event) => {
+      useBook.getState().setNotice(`PDF exported with warnings:\n${event.payload}`);
+    });
     return () => {
       unlisten.then((stop) => stop());
+      unlistenWarn.then((stop) => stop());
     };
   }, []);