feat: initial LexAI Chrome extension scaffold

- WXT + React 18 + TypeScript setup
- Background service worker (LLM API proxy)
- Content script (floating toolbar + text selection)
- Options page (provider/model/API key config)
- Popup UI
- Gitea Actions workflows (CI, preview, release)
- Vitest unit tests + Playwright E2E setup
- Supports: OpenAI, Anthropic, Groq, OpenRouter

wxt.config.ts

@@ -0,0 +1,19 @@
+import { defineConfig } from 'wxt';
+
+export default defineConfig({
+  extensionApi: 'chrome',
+  modules: ['@wxt-dev/module-react'],
+  manifest: {
+    name: 'LexAI - AI Writing Assistant',
+    description: 'Grammar checking and writing assistance powered by your own LLM API key',
+    version: '0.1.0',
+    permissions: [
+      'storage',
+      'activeTab',
+      'contextMenus',
+    ],
+    host_permissions: [
+      '<all_urls>',
+    ],
+  },
+});