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>

tests/contract/test_landing_routes.py

@@ -164,3 +164,19 @@ def test_get_landing_page_missing_slug(client, test_products):
     assert b'No Slug Product' not in response.data
     # But other active products should appear
     assert b'Apple Product' in response.data
+
+
+@pytest.mark.contract
+def test_landing_page_with_authenticated_user_renders_correctly(client, test_products, app):
+    """Test that landing page renders correctly - verifies url_for('landing.index') works
+
+    This test verifies that the base.html template references url_for('landing.index')
+    instead of url_for('index'), which would cause a BuildError.
+    """
+    # Just access the landing page - if url_for references are broken, this will fail
+    response = client.get('/')
+
+    assert response.status_code == 200
+    # Should contain products (proving the page rendered successfully)
+    assert b'Apple Product' in response.data
+    assert b'Zebra Product' in response.data