feat(payments): wallet screen with balances, benefits and storefront

Add the "Кошелёк" section to the settings hub: context-visible chip
balances, active benefits (no-ads term/forever, hints) and a storefront of
chip-priced values and money-priced chip packs. Guests have no wallet; the
Google Play build hides the money purchases behind a RuStore stub; a web
purchase that would draw VK/Telegram chips warns first.

Add the catalog read path the storefront needs — a context-projected
GET /api/v1/user/wallet/catalog (payments service + store, gateway op
wallet.catalog, FBS Catalog/CatalogProduct/CatalogAtom, client decode) —
plus the client leg for the existing wallet.get/buy ops. Value spends reuse
the existing spend path; the chip-pack purchase (money order flow) arrives
with payment intake, so its action is a disabled placeholder for now.

Covered by Go unit (catalog projection) + integration (/wallet/catalog over
Postgres), vitest (formatting, spendable selection, web-spend warning, GP
flag, codec + gateway encode round-trips) and Playwright mock e2e (render,
guest-hidden, GP stub, warning) on Chromium + WebKit.

backend/internal/payments/catalog_test.go

@@ -0,0 +1,136 @@
+package payments
+
+import (
+	"testing"
+
+	"github.com/google/uuid"
+)
+
+// catalog fixtures: a chip-priced value (hints), a multi-method chip pack, a VK-only chip pack, a
+// value with no CHIP price (misconfigured), and a pack whose only method is telegram.
+func fixtureCatalog() []catalogEntry {
+	value := catalogEntry{
+		id:    uuid.MustParse("00000000-0000-0000-0000-000000000001"),
+		title: "250 hints",
+		atoms: []atomQty{{atomType: "hints", quantity: 250}},
+		prices: []priceRow{
+			{method: "", currency: CurrencyChip, amount: 500},
+		},
+	}
+	pack := catalogEntry{
+		id:    uuid.MustParse("00000000-0000-0000-0000-000000000002"),
+		title: "100 chips",
+		atoms: []atomQty{{atomType: "chips", quantity: 100}},
+		prices: []priceRow{
+			{method: "vk", currency: CurrencyVote, amount: 20},
+			{method: "telegram", currency: CurrencyStar, amount: 25},
+			{method: "direct", currency: CurrencyRUB, amount: 14900},
+		},
+	}
+	vkOnlyPack := catalogEntry{
+		id:    uuid.MustParse("00000000-0000-0000-0000-000000000003"),
+		title: "VK-only chips",
+		atoms: []atomQty{{atomType: "chips", quantity: 50}},
+		prices: []priceRow{
+			{method: "vk", currency: CurrencyVote, amount: 10},
+		},
+	}
+	brokenValue := catalogEntry{
+		id:    uuid.MustParse("00000000-0000-0000-0000-000000000004"),
+		title: "no chip price",
+		atoms: []atomQty{{atomType: "noads_days", quantity: 30}},
+		// no CHIP price row — misconfigured, must be omitted
+	}
+	return []catalogEntry{value, pack, vkOnlyPack, brokenValue}
+}
+
+// byID indexes a projected storefront by product id for assertions.
+func byID(v CatalogView) map[string]CatalogProduct {
+	m := make(map[string]CatalogProduct, len(v.Products))
+	for _, p := range v.Products {
+		m[p.ProductID] = p
+	}
+	return m
+}
+
+func TestProjectCatalog_ContextMatrix(t *testing.T) {
+	const (
+		valueID  = "00000000-0000-0000-0000-000000000001"
+		packID   = "00000000-0000-0000-0000-000000000002"
+		vkPackID = "00000000-0000-0000-0000-000000000003"
+		brokenID = "00000000-0000-0000-0000-000000000004"
+	)
+	entries := fixtureCatalog()
+
+	tests := []struct {
+		name        string
+		cxt         Context
+		wantPackAmt int64
+		wantPackCur Currency
+		wantVKPack  bool // the vk-only pack visible?
+	}{
+		{"vk", Context{Kind: SourceVK}, 20, CurrencyVote, true},
+		{"vk-ios frozen still lists vk price", Context{Kind: SourceVK, Subtype: SubtypeIOS}, 20, CurrencyVote, true},
+		{"telegram", Context{Kind: SourceTelegram}, 25, CurrencyStar, false},
+		{"direct", Context{Kind: SourceDirect}, 14900, CurrencyRUB, false},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := byID(projectCatalog(entries, tc.cxt))
+
+			// The value shows everywhere at its uniform chip price, never a money price.
+			v, ok := got[valueID]
+			if !ok {
+				t.Fatalf("value missing from %s storefront", tc.name)
+			}
+			if v.Kind != "value" || v.Chips != 500 || v.MoneyCurrency != "" || v.MoneyAmount != 0 {
+				t.Errorf("value = %+v, want kind=value chips=500 no money", v)
+			}
+			if len(v.Atoms) != 1 || v.Atoms[0].AtomType != "hints" || v.Atoms[0].Quantity != 250 {
+				t.Errorf("value atoms = %+v, want [hints:250]", v.Atoms)
+			}
+
+			// The multi-method pack shows the context method's price, in that currency, no chips.
+			p, ok := got[packID]
+			if !ok {
+				t.Fatalf("pack missing from %s storefront", tc.name)
+			}
+			if p.Kind != "pack" || p.Chips != 0 || p.MoneyAmount != tc.wantPackAmt || p.MoneyCurrency != string(tc.wantPackCur) {
+				t.Errorf("pack = %+v, want kind=pack amount=%d currency=%s", p, tc.wantPackAmt, tc.wantPackCur)
+			}
+
+			// The vk-only pack shows only where a vk price exists.
+			if _, ok := got[vkPackID]; ok != tc.wantVKPack {
+				t.Errorf("vk-only pack present=%v, want %v (%s)", ok, tc.wantVKPack, tc.name)
+			}
+
+			// The misconfigured value (no CHIP price) is never shown.
+			if _, ok := got[brokenID]; ok {
+				t.Errorf("misconfigured value must be omitted (%s)", tc.name)
+			}
+		})
+	}
+}
+
+// An untrusted context (empty Kind) has no payment method, so it shows values only — no packs.
+func TestProjectCatalog_UntrustedShowsValuesOnly(t *testing.T) {
+	got := byID(projectCatalog(fixtureCatalog(), Context{}))
+	if _, ok := got["00000000-0000-0000-0000-000000000001"]; !ok {
+		t.Error("untrusted context should still list the chip-priced value")
+	}
+	for _, id := range []string{
+		"00000000-0000-0000-0000-000000000002",
+		"00000000-0000-0000-0000-000000000003",
+	} {
+		if _, ok := got[id]; ok {
+			t.Errorf("untrusted context must not list pack %s", id)
+		}
+	}
+}
+
+// An empty catalog projects to an empty storefront (no products seeded yet — the Release-1 state).
+func TestProjectCatalog_Empty(t *testing.T) {
+	if got := projectCatalog(nil, Context{Kind: SourceDirect}); len(got.Products) != 0 {
+		t.Errorf("empty catalog projected %d products, want 0", len(got.Products))
+	}
+}