chore: add unified ESLint config for TS, Astro, and JSX

- Introduce eslint.config.js with flat‑config structure
- Enable TypeScript linting using @typescript-eslint parser and plugin
- Add Astro parser/plugin with recommended rules
- Configure React/Preact JSX linting including react-hooks rules
- Apply project-aware parserOptions for TypeScript

eslint.config.js

@@ -0,0 +1,64 @@
+// eslint.config.js
+import js from "@eslint/js";
+import tseslint from "@typescript-eslint/eslint-plugin";
+import tsParser from "@typescript-eslint/parser";
+import astroPlugin from "eslint-plugin-astro";
+import astroParser from "astro-eslint-parser";
+import reactPlugin from "eslint-plugin-react";
+import reactHooks from "eslint-plugin-react-hooks";
+
+export default [
+  js.configs.recommended,
+
+  // TypeScript
+  {
+    files: ["**/*.ts", "**/*.tsx"],
+    languageOptions: {
+      parser: tsParser,
+      parserOptions: {
+        project: "./tsconfig.json",
+      },
+    },
+    plugins: {
+      "@typescript-eslint": tseslint,
+    },
+    rules: {
+      ...tseslint.configs.recommended.rules,
+    },
+  },
+
+  // Astro
+  {
+    files: ["**/*.astro"],
+    languageOptions: {
+      parser: astroParser,
+      parserOptions: {
+        parser: tsParser,
+      },
+    },
+    plugins: {
+      astro: astroPlugin,
+    },
+    rules: {
+      ...astroPlugin.configs.recommended.rules,
+    },
+  },
+
+  // JSX / Preact
+  {
+    files: ["**/*.jsx", "**/*.tsx"],
+    plugins: {
+      react: reactPlugin,
+      "react-hooks": reactHooks,
+    },
+    rules: {
+      ...reactPlugin.configs.recommended.rules,
+      ...reactHooks.configs.recommended.rules,
+    },
+    settings: {
+      react: {
+        version: "detect",
+      },
+    },
+  },
+];