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>

vitest.config.ts

@@ -1,6 +1,12 @@
+import { fileURLToPath } from 'node:url';
 import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
+  resolve: {
+    alias: {
+      '@lib': fileURLToPath(new URL('./src/lib', import.meta.url)),
+    },
+  },
   test: {
     environment: 'jsdom',
     globals: true,