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.

manifest/firefox.json

@@ -0,0 +1,25 @@
+{
+    "manifest_version": 2,
+    "name": "AutistMask",
+    "version": "0.1.0",
+    "description": "Minimal Ethereum wallet for Firefox",
+    "permissions": ["storage", "activeTab"],
+    "browser_action": {
+        "default_popup": "src/popup/index.html"
+    },
+    "background": {
+        "scripts": ["src/background/index.js"]
+    },
+    "content_scripts": [
+        {
+            "matches": ["<all_urls>"],
+            "js": ["src/content/index.js"],
+            "run_at": "document_start"
+        }
+    ],
+    "browser_specific_settings": {
+        "gecko": {
+            "id": "autistmask@sneak.berlin"
+        }
+    }
+}