Extract project header, share link, exports, and auth status

- ProjectHeader component replaces the headerButtons JSX in context; it
  owns the export menu and share panel state.
- useShareLink (share status/enable/revoke/copy) and useSharedView
  (?share= read-only view, now used directly by the router).
- utils/projectExport: HTML/Markdown/DOCX download and print-to-PDF.
- AuthStatus component replaces the authStatus JSX in context.
- Pages import wordStudy helpers directly instead of via context.
- Remove dead copyPronunciationGuide (PronunciationPage has its own).
- Tests: Markdown export, enabling a share link, the ?share= view.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/pages/SettingsPage.jsx

@@ -1,5 +1,6 @@
 import { useState } from 'react';
 import { useApp } from '../context/AppContext.js';
+import AuthStatus from '../components/AuthStatus.jsx';
 import {
   startMfaSetup,
   confirmMfaSetup,
@@ -11,7 +12,6 @@ import {
 export default function SettingsPage() {
   const {
     authUser, setAuthUser,
-    authStatus,
     goHome,
   } = useApp();
   const [mfaSetup, setMfaSetup] = useState(null); // { secret, qrCodeDataUrl } while setup is in progress
@@ -123,7 +123,7 @@ export default function SettingsPage() {
             >
               ← Back
             </button>
-            {authStatus}
+            <AuthStatus />
           </div>
         </div>
       </header>