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>
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