fix: help page back link preserves active persona

Pass ?persona= query param on the help link so the server knows which
persona to return to. Previously always defaulted to personas[0], causing
navigation back to the wrong persona.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/index.html

@@ -59,7 +59,7 @@
                     </form>
                 </div>
             </div>
-            <a href="/help" class="hdr-btn" title="Help &amp; reference" style="text-decoration:none">
+            <a id="help-link" href="/help" class="hdr-btn" title="Help &amp; reference" style="text-decoration:none">
                 <svg data-lucide="circle-help" class="btn-icon"></svg>
             </a>
         </nav>