Add ESLint configuration for JavaScript files

This introduces a flat ESLint configuration for JavaScript,
MJS, and CJS files, including recommended rules and global
variables for both browser and Node.js environments.

eslint.config.js

@@ -0,0 +1,44 @@
+// ESLint 9+ flat config format (global default)
+// See: https://eslint.org/docs/latest/use/configure/configuration-files
+
+export default [
+  {
+    files: ["**/*.js", "**/*.mjs", "**/*.cjs"],
+    languageOptions: {
+      ecmaVersion: "latest",
+      sourceType: "module",
+      globals: {
+        // Browser globals
+        console: "readonly",
+        window: "readonly",
+        document: "readonly",
+        navigator: "readonly",
+        // Node.js globals
+        process: "readonly",
+        __dirname: "readonly",
+        __filename: "readonly",
+        require: "readonly",
+        module: "readonly",
+        exports: "readonly",
+        global: "readonly",
+        Buffer: "readonly",
+      },
+    },
+    rules: {
+      // Recommended rules
+      "no-undef": "error",
+      "no-unused-vars": "warn",
+      "no-unreachable": "warn",
+      "no-constant-condition": "error",
+      "no-duplicate-case": "error",
+      
+      // Best practices
+      "eqeqeq": ["warn", "always"],
+      "no-implicit-globals": "warn",
+      
+      // Style (minimal)
+      "semi": ["warn", "always"],
+      "quotes": ["warn", "double", { "avoidEscape": true }],
+    },
+  },
+];