chore(catalog): order the admin catalog list like the public offer

Sales (chip packs) first, ascending by rouble price; then the chip-exchange
values, grouped and price-sorted the same way projectOfferPricing lists them, so
the /catalog console mirrors what a buyer sees. Internal cosmetics only — no
product behaviour change. The value-group order moves to a shared helper
(valueGroup) so the offer and the admin list cannot drift.

backend/internal/payments/catalog_test.go

@@ -134,3 +134,47 @@ func TestProjectCatalog_Empty(t *testing.T) {
 		t.Errorf("empty catalog projected %d products, want 0", len(got.Products))
 	}
 }
+
+// TestSortAdminCatalog checks the admin list is ordered like the public offer: chip packs first,
+// ascending by rouble price; then values, grouped (hints only -> no-ads only -> no-ads + hints) and
+// ascending by chip price within a group. Stable and applied to active + archived alike.
+func TestSortAdminCatalog(t *testing.T) {
+	pack := func(title string, rub int64) AdminProduct {
+		return AdminProduct{
+			Title:  title,
+			Atoms:  []AtomLine{{Atom: atomChips, Quantity: 1}},
+			Prices: []PriceLine{{Method: string(SourceDirect), Currency: CurrencyRUB, Amount: rub}},
+		}
+	}
+	value := func(title string, chips int64, atoms ...string) AdminProduct {
+		p := AdminProduct{Title: title, Prices: []PriceLine{{Currency: CurrencyChip, Amount: chips}}}
+		for _, a := range atoms {
+			p.Atoms = append(p.Atoms, AtomLine{Atom: a, Quantity: 1})
+		}
+		return p
+	}
+	products := []AdminProduct{
+		pack("packDear", 30000),
+		value("bundle", 500, "hints", "noads_days"),
+		pack("packCheap", 10000),
+		value("adsOnly", 150, "noads_days"),
+		value("hintsBig", 200, "hints"),
+		value("hintsSmall", 50, "hints"),
+	}
+	SortAdminCatalog(products)
+	want := []string{"packCheap", "packDear", "hintsSmall", "hintsBig", "adsOnly", "bundle"}
+	for i, p := range products {
+		if p.Title != want[i] {
+			t.Fatalf("order[%d] = %q, want %q (full: %v)", i, p.Title, want[i], titles(products))
+		}
+	}
+}
+
+// titles extracts the product titles for a failure message.
+func titles(products []AdminProduct) []string {
+	out := make([]string, len(products))
+	for i, p := range products {
+		out[i] = p.Title
+	}
+	return out
+}