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/model/fonts.ts

@@ -54,6 +54,14 @@ export function bundledFont(id: string): BundledFont | undefined {
   return BUNDLED_FONTS.find((f) => f.id === id);
 }
 
+export function encodeRef(ref: FontRef): string {
+  return ref.kind === "bundled" ? `b:${ref.id}` : `s:${ref.family}`;
+}
+
+export function decodeRef(value: string): FontRef {
+  return value.startsWith("s:") ? { kind: "system", family: value.slice(2) } : { kind: "bundled", id: value.slice(2) };
+}
+
 export function fontFamilyName(ref: FontRef): string {
   if (ref.kind === "system") return ref.family;
   return bundledFont(ref.id)?.family ?? "Literata";