Add PUBLIC_DIR env var to merge external public files at build time

Files in the external directory take precedence over the built-in
public/ directory. Works in both dev (middleware) and build (post-copy)
modes. Also fix missing magicMouse property in TestApp.

vite.config.ts

@@ -2,6 +2,7 @@ import fs from "node:fs";
 import path from "node:path";
 
 import react from "@vitejs/plugin-react-swc";
+import type { Plugin } from "vite";
 import { defineConfig } from "vitest/config";
 
 import { DittoConfigSchema } from "./src/lib/schemas";
@@ -27,7 +28,70 @@ function loadDittoConfig(): object | undefined {
   return result;
 }
 
+/**
+ * Copy all files from `src` into `dest`, overwriting existing files.
+ * Recursively handles subdirectories.
+ */
+function copyDirSync(src: string, dest: string): void {
+  fs.mkdirSync(dest, { recursive: true });
+  for (const entry of fs.readdirSync(src, { withFileTypes: true })) {
+    const srcPath = path.join(src, entry.name);
+    const destPath = path.join(dest, entry.name);
+    if (entry.isDirectory()) {
+      copyDirSync(srcPath, destPath);
+    } else {
+      fs.copyFileSync(srcPath, destPath);
+    }
+  }
+}
+
+/**
+ * Vite plugin that merges an external public directory on top of the default one.
+ * Set the PUBLIC_DIR env var to a directory path. Files in that directory take
+ * precedence over files in the built-in `public/` directory.
+ *
+ * - In build mode, files are copied into the output after the default public dir.
+ * - In dev mode, the external directory is served with higher priority.
+ */
+function mergePublicDir(externalDir: string): Plugin {
+  const resolved = path.resolve(externalDir);
+
+  return {
+    name: "ditto:merge-public-dir",
+
+    configureServer(server) {
+      // Serve files from the external public dir before the default public dir.
+      server.middlewares.use((req, res, next) => {
+        if (!req.url) return next();
+
+        const urlPath = decodeURIComponent(new URL(req.url, "http://localhost").pathname);
+        const filePath = path.join(resolved, urlPath);
+
+        try {
+          const stat = fs.statSync(filePath);
+          if (stat.isFile()) {
+            // Let Vite's static middleware handle it by pointing to the file.
+            const stream = fs.createReadStream(filePath);
+            stream.pipe(res);
+            return;
+          }
+        } catch {
+          // File not found in external dir — fall through to default public dir
+        }
+
+        next();
+      });
+    },
+
+    writeBundle(options) {
+      const outDir = options.dir ?? path.resolve("dist");
+      copyDirSync(resolved, outDir);
+    },
+  };
+}
+
 const dittoConfig = loadDittoConfig();
+const publicDir = process.env.PUBLIC_DIR;
 
 // https://vitejs.dev/config/
 export default defineConfig(() => ({
@@ -37,6 +101,7 @@ export default defineConfig(() => ({
   },
   plugins: [
     react(),
+    ...(publicDir ? [mergePublicDir(publicDir)] : []),
   ],
   define: {
     __DITTO_CONFIG__: JSON.stringify(dittoConfig ?? null),