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>

entrypoints/background.ts

@@ -1,5 +1,6 @@
 import { defineBackground } from 'wxt/utils/define-background';
 import nacl from 'tweetnacl';
+import type { AnalyzePayload, LexAIConfig, LexAIResponse } from '@lib/types';
 
 // ─── Fetch with timeout ───────────────────────────────────────────────────────
 
@@ -13,27 +14,6 @@ async function fetchWithTimeout(url: string, options: RequestInit, timeoutMs = 3
   }
 }
 
-// ─── Types ────────────────────────────────────────────────────────────────────
-
-interface AnalyzePayload {
-  text: string;
-  action: string;
-  style?: string;
-}
-
-interface LexAIConfig {
-  provider?: string;
-  apiKey?: string;
-  apiKeyEnc?: string;
-  encKey?: string;
-  model?: string;
-}
-
-interface LexAIResponse {
-  result?: string;
-  error?: string;
-}
-
 // ─── System prompts ───────────────────────────────────────────────────────────
 
 function getSystemPrompt(action: string, style?: string): string {