Fix url_for references and improve navigation

Issues fixed:
- Fixed base.html to use url_for('landing.index') instead of url_for('index')
- Fixed error_403.html and error_404.html to use correct landing page endpoint
- Added Dashboard link in base.html nav for product owners
- Removed obsolete templates/index.html (replaced by landing page)
- Added dashboard link in landing/index.html for authenticated product owners
- Added test to verify landing page renders correctly with proper url_for references

All 8 landing page tests passing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/templates/error_404.html

@@ -8,7 +8,7 @@
     <p>The page or resource you requested could not be found.</p>
     <p>
         <a href="{{ url_for('dashboard.list') }}">Return to Dashboard</a> |
-        <a href="{{ url_for('index') }}">Go to Home</a>
+        <a href="{{ url_for('landing.index') }}">Go to Home</a>
     </p>
 </div>
 {% endblock %}