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-tauri/src/lib.rs

@@ -1,4 +1,5 @@
 mod epub;
+mod fonts;
 mod library;
 mod pdf;
 mod project;
@@ -83,6 +84,7 @@ pub fn run() {
         .invoke_handler(tauri::generate_handler![
             epub::package_epub,
             epub::unzip_epub,
+            fonts::list_system_fonts,
             pdf::compile_pdf,
             project::read_file,
             project::write_file,