Stack low-stock badge with discount badge instead of hiding it, update READMEs

Ausverkauft/discount/low-stock badges used a single either/or slot, so a
product with an active discount silently never showed its low-stock pill
(caught live: todo-karten had both at once). Badges now stack in a flex
column across ProductGrid/ProductSpotlight/RelatedProducts/Pricing, with
Ausverkauft still winning outright. READMEs updated for this and the
recent discount-field gating, shipping-address, and money-rounding changes.

app/todo-cards/components/Pricing.tsx

@@ -48,15 +48,20 @@ export async function Pricing() {
             <span className="absolute top-3 left-3 rounded-full bg-text-muted px-2.5 py-1 text-label font-bold text-bg-base">
               Ausverkauft
             </span>
-          ) : discount !== null ? (
-            <span className="absolute top-3 left-3 rounded-full bg-brand px-2.5 py-1 text-label font-bold text-text-primary">
-              -{discount}%
-            </span>
           ) : (
-            anyLowStock && (
-              <span className="absolute top-3 left-3 rounded-full bg-warning px-2.5 py-1 text-label font-bold text-text-on-dark">
-                Nur noch wenige verfügbar
-              </span>
+            (discount !== null || anyLowStock) && (
+              <div className="absolute top-3 left-3 flex flex-col items-start gap-1">
+                {discount !== null && (
+                  <span className="rounded-full bg-brand px-2.5 py-1 text-label font-bold text-text-primary">
+                    -{discount}%
+                  </span>
+                )}
+                {anyLowStock && (
+                  <span className="rounded-full bg-warning px-2.5 py-1 text-label font-bold text-text-on-dark">
+                    Nur noch wenige verfügbar
+                  </span>
+                )}
+              </div>
             )
           )}
         </div>