test: let vitest import svelte rune modules

Adds the svelte plugin and the $lib alias so *.svelte.js modules can be
imported by tests. The lesson interaction logic was untestable without it.

vitest.config.js

@@ -1,6 +1,18 @@
+import { fileURLToPath } from 'node:url';
 import { defineConfig } from 'vitest/config';
+import { svelte } from '@sveltejs/vite-plugin-svelte';
 
 export default defineConfig({
+  // The svelte plugin is what lets `*.svelte.js` rune modules be imported by
+  // tests; without it the interaction modules are untestable.
+  plugins: [svelte()],
+  resolve: {
+    conditions: ['browser'],
+    // SvelteKit normally injects $lib; vitest runs without that layer.
+    alias: {
+      $lib: fileURLToPath(new URL('./src/lib', import.meta.url)),
+    },
+  },
   test: {
     include: ['src/**/*.test.js'],
     environment: 'node',