feat: shared nav bar on Help and Settings pages

Replaces the lone "← Back to Cortex" link with a consistent page-nav
on both pages: ← Chat | Help | Settings | Sign out

Active page is highlighted purple; others are muted gray.
Settings page gets a {{ help_href }} template var from settings.py.
Help page builds nav links from the existing cfg JS object.

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

cortex/routers/settings.py

@@ -72,6 +72,7 @@ def _settings_page(username: str, personas: list[str], success: str = "", error:
     html = html.replace("{{ persona_items }}", persona_items or "<li><em>No personas yet.</em></li>")
     back_persona = personas[0] if personas else ""
     html = html.replace("{{ back_href }}", f"/{username}/{back_persona}" if back_persona else "/")
+    html = html.replace("{{ help_href }}", f"/help?persona={back_persona}" if back_persona else "/help")
     if success:
         html = html.replace("<!-- SUCCESS -->", f'<p class="success">{success}</p>')
     if error: