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>

.gitea/workflows/lint.yml

@@ -0,0 +1,20 @@
+name: Lint
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+    branches: ["**"]
+
+jobs:
+  js-lint:
+    name: JS (eslint)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install ESLint
+        run: npm install --save-dev eslint@8
+
+      - name: Run ESLint
+        run: npx eslint --ext .js base.js node/