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

tests/unit/setup.ts

@@ -0,0 +1,20 @@
+// Mock Chrome extension APIs
+global.chrome = {
+  storage: {
+    local: {
+      get: vi.fn((keys, cb) => cb({})),
+      set: vi.fn((data, cb) => cb && cb()),
+    },
+    sync: {
+      get: vi.fn((keys, cb) => cb({})),
+      set: vi.fn((data, cb) => cb && cb()),
+    },
+  },
+  runtime: {
+    sendMessage: vi.fn(),
+    onMessage: {
+      addListener: vi.fn(),
+    },
+    openOptionsPage: vi.fn(),
+  },
+} as any;