Fix 500: avoid dict lookup as default filter argument

Django raises VariableDoesNotExist when a failed dict lookup is used as a
filter argument (|default:mineral.properties_ru.X). Rewrite color description
to use an {% if %} block so the dict lookup is always the main variable.

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

dailystone/templates/dailystone/stone.html

@@ -534,7 +534,9 @@
             <div class="color-swatch"></div>
             <div class="color-info">
                 <span class="lang-en">{{ mineral.color_description|default:"Typical color" }}</span>
-                <span class="lang-ru">{{ mineral.color_description_ru|default:mineral.properties_ru.color_description|default:mineral.color_description|default:"Типичный цвет" }}</span>
+                <span class="lang-ru">
+                    {% if mineral.color_description_ru %}{{ mineral.color_description_ru }}{% else %}{{ mineral.properties_ru.color_description|default:mineral.color_description|default:"Типичный цвет" }}{% endif %}
+                </span>
                 <br><span class="hex">{{ mineral.color_hex }}</span>
             </div>
         </div>