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/content.ts

@@ -1,4 +1,5 @@
 import { defineContentScript } from 'wxt/utils/define-content-script';
+import { isExtensionValid, safeSendMessage } from '@lib/messaging';
 
 export default defineContentScript({
   matches: ['<all_urls>'],
@@ -362,15 +363,7 @@ export default defineContentScript({
       }
     }
 
-    // ─── Extension context guard ──────────────────────────────────────────────
-
-    function isExtensionValid(): boolean {
-      try {
-        return typeof chrome !== 'undefined' && !!chrome.runtime?.id;
-      } catch {
-        return false;
-      }
-    }
+    // Extension context guard: isExtensionValid is imported from ~/lib/messaging.
 
     function showErrorToast(msg: string) {
       const toast = document.createElement('div');
@@ -528,7 +521,7 @@ export default defineContentScript({
           grid.style.display       = 'none';
           spinnerSlot.style.display = 'flex';
 
-          const response = await safeSendMessage({
+          const response = await sendToBackground({
             type:   'COPY_AS',
             text,
             format: fmt,
@@ -563,22 +556,13 @@ export default defineContentScript({
     }
 
     // ─── Safe chrome.runtime.sendMessage wrapper ──────────────────────────────
+    // Shared implementation; on a stale extension context we toast and clean up.
 
-    async function safeSendMessage(payload: Record<string, unknown>): Promise<unknown> {
-      if (!isExtensionValid()) {
+    async function sendToBackground(payload: Record<string, unknown>): Promise<unknown> {
+      return safeSendMessage(payload, () => {
         showErrorToast('LexAI was updated — please refresh this page.');
-        return null;
-      }
-      try {
-        return await chrome.runtime.sendMessage(payload);
-      } catch (err) {
-        if (String(err).includes('Extension context invalidated') ||
-            String(err).includes('message channel closed')) {
-          showErrorToast('LexAI was updated — please refresh this page.');
-          hideToolbar();
-        }
-        return null;
-      }
+        hideToolbar();
+      });
     }
 
     // ─── LLM call ─────────────────────────────────────────────────────────────
@@ -628,14 +612,14 @@ export default defineContentScript({
       }
 
       try {
-        const response = await safeSendMessage({
+        const response = await sendToBackground({
           type:    'ANALYZE_TEXT',
           payload: { text: textToProcess, action, style: currentStyle },
         }) as { error?: string; result?: string } | null;
 
         hideToolbar();
 
-        if (response === null) return; // safeSendMessage already handled the error
+        if (response === null) return; // sendToBackground already handled the error
 
         if (response?.error) {
           showModal(`❌ ${response.error}`, null, snapStart, snapEnd, snapElement, snapRange, action, textToProcess);
@@ -832,7 +816,7 @@ export default defineContentScript({
         regenBtn.disabled = true;
         styleSelect.disabled = true;
 
-        const response = await safeSendMessage({
+        const response = await sendToBackground({
           type:    'ANALYZE_TEXT',
           payload: { text: textForRegenerate, action, style: chosenStyle },
         }) as { error?: string; result?: string } | null;