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/export/run.ts

@@ -1,5 +1,6 @@
 import type { Book } from "../model/book";
 import { exportEpub, exportPdf } from "./exporters";
+import { unsupportedScripts } from "./typst";
 import { useBook } from "../store/useBook";
 import { isDesktop } from "../ipc";
 
@@ -29,6 +30,11 @@ export async function runExport(format: "pdf" | "epub"): Promise<void> {
   await nextPaint();
   try {
     await run(book);
+    if (format === "pdf") {
+      const scripts = unsupportedScripts(book);
+      if (scripts.length)
+        setNotice(`Some ${scripts.join(", ")} characters may not render — the PDF fonts cover Latin scripts only.`);
+    }
   } catch (e) {
     setNotice(`${label} export failed: ${e}`);
   } finally {