Fix product slug resolution: fall back to g.post_slug

App-level url_value_preprocessor pops slug into g.post_slug before
the blueprint preprocessor runs, leaving values empty. Fall back to
g.post_slug so resolve_product and cart route get the slug.

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

bp/product/routes.py

@@ -25,7 +25,9 @@ def register():
     bp = Blueprint("product", __name__, url_prefix="/product/<slug>")
     @bp.url_value_preprocessor
     def pull_blog(endpoint, values):
-      g.product_slug = values.get("slug")
+      # App-level preprocessor pops "slug" into g.post_slug before this runs,
+      # so values.get("slug") is None. Fall back to g.post_slug.
+      g.product_slug = values.get("slug") or getattr(g, "post_slug", None)
 
     # ─────────────────────────────────────────────────────────────
     # BEFORE REQUEST: Slug or numeric ID resolver