fix: keep the interface visible while fonts load

Render fallback fonts immediately and preload regular Maple in the background. Cache public fonts after use without eagerly downloading every font during PWA installation.

web/demo.html

@@ -1,5 +1,5 @@
 <!doctype html>
-<html lang="en" data-rm-fonts="loading">
+<html lang="en">
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />