Build: vite.worker.format='es' so render-worker can import shared modules

Default Vite worker format is `iife`, which forbids code splitting; the
score render worker imports renderScoreUncached from render.ts, which
gets code-split, so the build broke with:

  Invalid value "iife" for option "worker.format" - UMD and IIFE
  output formats are not supported for code-splitting builds.

ES module workers fix this and are supported in every browser our
runtime guard already checks for (the worker is feature-detected and
falls back to main-thread render on older targets).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: RuitingMa

astro.config.mjs

@@ -9,4 +9,13 @@ export default defineConfig({
   // Order: mdx first so .mdx essays can import components; font-subset runs
   // at astro:build:done, after everything else has emitted HTML.
   integrations: [mdx(), fontSubset()],
+  vite: {
+    worker: {
+      // ES module workers — required because src/lib/score/render-worker.ts
+      // imports from render.ts (which is code-split). Vite's default `iife`
+      // worker format would refuse the import; `es` lets the worker chunk
+      // share modules with the main bundle.
+      format: 'es',
+    },
+  },
 });