fix: remove SPA fallback so prerendered HTML wins at /

With prerender=true on +layout.js, adapter-static emits index.html
per route. Setting fallback: 'index.html' overwrites the prerendered
home with the SPA bootstrap shell, leaving the live page empty before
hydration. Drop the fallback option.

svelte.config.js

@@ -7,7 +7,7 @@ const base = process.env.SITE_BASE ?? '/mathmax';
 const config = {
   preprocess: vitePreprocess(),
   kit: {
-    adapter: adapter({ fallback: 'index.html' }),
+    adapter: adapter(),
     paths: { base },
   },
 };