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/app.js

@@ -32,6 +32,10 @@
 
         if (headerEmoji) headerEmoji.textContent = CORTEX_EMOJI;
 
+        // Wire help link to preserve current persona on return
+        const helpLink = document.getElementById('help-link');
+        if (helpLink) helpLink.href = `/help?persona=${encodeURIComponent(CORTEX_PERSONA)}`;
+
         let sessionId        = null;
         let primaryBackend   = 'claude';
         let activeController = null;