feat: promote a reference as the primary shown in lists

Add an is_primary flag to references; the promoted ref sorts first on the item page and is the one shown under the item in the list and catalog. Item page gets a star toggle per reference.

1 files changed, 3 insertions, 1 deletions

diff --git a/README.md b/README.md
index 0eed298..b047498 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,9 @@ chores).
   refresh to see your partner's changes.
 - **Product references** — for items like "coffee" or "pasta": one or more
   reference brands with a price per unit (€/kg or €/L) and a quality score
-  (0–100), comparable from the item while in the store.
+  (0–100), comparable from the item while in the store. One reference can be
+  promoted as the item's primary — it is listed first and is the one shown
+  under the item in the catalog and shopping list.
 - **Categories** — items assigned to store aisles, drag to reorder the list
   to match the physical layout.
 - **Projects & routines** — projects ("Household", "Personal growth", …)