fix(loading): add aria-label for accessibility

Add aria-label to loading spinner image for screen reader accessibility while keeping empty alt text to prevent fallback display if image fails to load.

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

Co-Authored-By: Claude <[email protected]>

Author: eschutho

superset/templates/superset/spa.html

@@ -100,6 +100,7 @@
           <img
             src="{{ tokens.brandSpinnerUrl }}"
             alt=""
+            aria-label="Loading"
             style="{{ spinner_style }}"
           />
         {% endif %}