fix(bigmind): render achievement card background images via inline style

The .ach-image div had correct CSS dimensions (64x64) and background-size:cover
but was missing the inline style="background-image: url(...)" — so the div
rendered as an empty circle. Fixed by extracting img_url variable and applying
it as style attribute in the f-string. All 39 achievement PNGs now load.

303/303 tests passing.

mcp/bigmind/bigmind/web_render.py

@@ -33,7 +33,8 @@ def _render_achievements(achievements: list) -> str:
 
         if a.get("image"):
             tier = a["id"].rsplit("_", 1)[-1]
-            visual_html = f'<div class="ach-image tier-{tier}">{lock_overlay}</div>'
+            img_url = _esc(a["image"])
+            visual_html = f'<div class="ach-image tier-{tier}" style="background-image: url({img_url});">{lock_overlay}</div>'
         else:
             visual_html = f'<div class="ach-icon">{a["icon"]}{lock_overlay}</div>'