ci: add ESLint lint workflow with per-directory configs

Node.js env for server.js/worker/, browser env for public/.
All errors downgraded to warnings (empty blocks, inner declarations,
loose equality, useless escape, constant condition) for practical CI.

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

.eslintrc.json

@@ -0,0 +1,13 @@
+{
+    "env": { "node": true, "es2021": true },
+    "extends": "eslint:recommended",
+    "parserOptions": { "ecmaVersion": 2021, "sourceType": "commonjs" },
+    "rules": {
+        "no-unused-vars": "warn",
+        "no-empty": "warn",
+        "no-constant-condition": "warn",
+        "no-useless-escape": "warn",
+        "semi": ["error", "always"],
+        "eqeqeq": "warn"
+    }
+}