feat(fonts): add font picker to book setup

Add a Typography section to book setup with one-click preset pairings,
an advanced body/heading picker, and a live sample. Enumerate installed
fonts via a new list_system_fonts Tauri command (fontdb) so they appear
alongside the bundled set, with a note that system fonts may not embed
in exports.

src/ipc.ts

@@ -15,3 +15,12 @@ export async function compilePdf(
   const buffer = await invoke<ArrayBuffer>("compile_pdf", { source, images, emitWarnings });
   return new Uint8Array(buffer);
 }
+
+export async function listSystemFonts(): Promise<string[]> {
+  if (!isDesktop) return [];
+  try {
+    return await invoke<string[]>("list_system_fonts");
+  } catch {
+    return [];
+  }
+}