Add ESLint config enforcing no-undef and eqeqeq

Without a config file, ESLint was running with no-undef disabled, meaning
undefined variable references in static/app.js were silently ignored.
Add .eslintrc.json with no-undef: error and eqeqeq: error so CI actually
catches JS bugs.

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

.eslintrc.json

@@ -0,0 +1,21 @@
+{
+  "env": {
+    "browser": true,
+    "es2021": true
+  },
+  "globals": {
+    "lt": "readonly",
+    "GANDALF_CONFIG": "readonly",
+    "CSS": "readonly"
+  },
+  "rules": {
+    "no-undef": "error",
+    "no-unused-vars": ["warn", { "argsIgnorePattern": "^_", "varsIgnorePattern": "^_" }],
+    "no-console": "off",
+    "eqeqeq": ["error", "always", { "null": "ignore" }]
+  },
+  "parserOptions": {
+    "ecmaVersion": 2021,
+    "sourceType": "script"
+  }
+}