feat(fonts): honor selected fonts in PDF export

Interpolate the book's body and heading families into the Typst
preamble, title page, cover, and contents. Embed the needed bundled
fonts and load any chosen system fonts (via fontdb) into the Typst
engine so PDF preview and export match the on-screen fonts.

src/components/Dock.tsx

@@ -107,8 +107,8 @@ function PdfDock() {
     clearTimeout(timer.current);
     setBusy(true);
     timer.current = setTimeout(() => {
-      const { source, images } = coverActive ? coverToPdfInputs(book) : chapterToPdfInputs(book, idx);
-      compilePdf(source, images)
+      const { source, images, fonts } = coverActive ? coverToPdfInputs(book) : chapterToPdfInputs(book, idx);
+      compilePdf(source, images, false, fonts)
         .then((bytes) => {
           setPdf(bytes);
           setError(null);