ci: add ESLint workflow

- .gitea/workflows/lint.yml: lint base.js and node/ directory
- .eslintrc.json: browser + node environment, CommonJS
- .gitignore: ignore node_modules/ and .env
- package.json + package-lock.json: eslint@8 dev dependency

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.eslintrc.json

@@ -0,0 +1,11 @@
+{
+    "env": { "browser": true, "node": true, "es2021": true },
+    "parserOptions": { "ecmaVersion": 2021, "sourceType": "script" },
+    "rules": {
+        "no-unused-vars": "warn",
+        "no-undef": "warn",
+        "no-empty": "warn",
+        "semi": ["error", "always"],
+        "eqeqeq": "warn"
+    }
+}