refactor: extract shared types + safe chrome wrappers into src/lib

- src/lib/types.ts: message contract (both ANALYZE_TEXT shapes preserved),
  config/response types, storage-key constants.
- src/lib/messaging.ts: single safeStorageGet/safeStorageSet/safeSendMessage/
  isExtensionValid implementation replacing the three divergent copies in
  Options, Popup, and content. Content script keeps its refresh-toast
  behavior via an onContextInvalidated callback.
- New '@lib' import alias (wxt force-overwrites '~' and '@' to srcDir, so
  those cannot point at ./src); wired in wxt.config, tsconfig, vitest.
- tests/unit/messaging.test.ts: 14 unit tests over the wrappers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

wxt.config.ts

@@ -1,8 +1,14 @@
+import { resolve } from 'node:path';
 import { defineConfig } from 'wxt';
 import pkg from './package.json';
 
 export default defineConfig({
   extensionApi: 'chrome',
+  // Shared-code alias. Deliberately NOT "~" or "@" — WXT force-overwrites those
+  // to srcDir (the project root here), so they cannot point at ./src.
+  alias: {
+    '@lib': resolve(__dirname, 'src/lib'),
+  },
   modules: ['@wxt-dev/module-react'],
   manifest: {
     name: 'LexAI - AI Writing Assistant',