Fix remaining url_for('index') references

Fixed the remaining url_for('index') calls that were causing test failures:
- app/routes/auth.py: Updated logout redirect to use landing.index
- app/templates/auth/login.html: Updated "Return to home" link to use landing.index
- app/templates/base.html: Removed administrator placeholder text (cleaned up)

All url_for('index') references have been replaced with url_for('landing.index').

Test results: 57 passed, 1 skipped, 4 errors (pre-existing fixture issues in performance tests)

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

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

app/templates/auth/login.html

@@ -22,6 +22,6 @@
 </form>
 
 <p style="margin-top: 20px;">
-    <a href="{{ url_for('index') }}">Return to home page</a>
+    <a href="{{ url_for('landing.index') }}">Return to home page</a>
 </p>
 {% endblock %}