Add project scaffolding

Makefile, Dockerfile, CI workflow, prettier config, manifests for
Chrome (MV3) and Firefox (MV2), source directory structure, and
minimal test suite. All checks pass.

build.js

@@ -0,0 +1,46 @@
+const fs = require("fs");
+const path = require("path");
+
+const DIST_CHROME = path.join(__dirname, "dist", "chrome");
+const DIST_FIREFOX = path.join(__dirname, "dist", "firefox");
+
+function ensureDir(dir) {
+    fs.mkdirSync(dir, { recursive: true });
+}
+
+function copyDir(src, dest) {
+    ensureDir(dest);
+    const entries = fs.readdirSync(src, { withFileTypes: true });
+    for (const entry of entries) {
+        const srcPath = path.join(src, entry.name);
+        const destPath = path.join(dest, entry.name);
+        if (entry.isDirectory()) {
+            copyDir(srcPath, destPath);
+        } else {
+            fs.copyFileSync(srcPath, destPath);
+        }
+    }
+}
+
+function build() {
+    console.log("Building AutistMask extension...");
+
+    ensureDir(DIST_CHROME);
+    ensureDir(DIST_FIREFOX);
+
+    copyDir(path.join(__dirname, "src"), path.join(DIST_CHROME, "src"));
+    copyDir(path.join(__dirname, "src"), path.join(DIST_FIREFOX, "src"));
+
+    fs.copyFileSync(
+        path.join(__dirname, "manifest", "chrome.json"),
+        path.join(DIST_CHROME, "manifest.json"),
+    );
+    fs.copyFileSync(
+        path.join(__dirname, "manifest", "firefox.json"),
+        path.join(DIST_FIREFOX, "manifest.json"),
+    );
+
+    console.log("Build complete: dist/chrome/ and dist/firefox/");
+}
+
+build();