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/hooks/useSharedView.js

@@ -0,0 +1,20 @@
+// A ?share=TOKEN URL shows a read-only view of someone else's project. No
+// login is needed, so this runs independently of auth.
+import { useEffect, useState } from 'react';
+import { getSharedProject } from '../syncService.js';
+
+export function useSharedView() {
+  const [token] = useState(() => new URLSearchParams(window.location.search).get('share'));
+  const [project, setProject] = useState(null);
+  const [error, setError] = useState('');
+
+  useEffect(() => {
+    if (!token) return;
+    getSharedProject(token).then((result) => {
+      if (result.ok) setProject(result.data);
+      else setError(result.error ?? 'This share link is invalid or has been revoked.');
+    });
+  }, [token]);
+
+  return { token, project, error };
+}