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.

ui/src/lib/distribution.ts

@@ -0,0 +1,21 @@
+// Distribution channel of the native build. Google Play forbids selling in-app currency through
+// an external gate, so the Google Play build hides the purchase actions and shows a stub pointing
+// the user to the RuStore build; every other build (web, VK, Telegram, RuStore native) sells
+// normally. The channel is a build-time flag the Google Play build sets, not a runtime guess.
+
+/**
+ * isGooglePlayBuild reports whether this is the Google Play native build, where in-app-currency
+ * purchases are hidden. It reads the build-time flag VITE_GP_BUILD (set to "1" only by the Google
+ * Play build); every other build reads false. Under the mock e2e it can be forced on with a `?gp`
+ * query parameter so the stub path is exercisable without a separate build.
+ */
+export function isGooglePlayBuild(): boolean {
+  if (
+    import.meta.env.MODE === 'mock' &&
+    typeof window !== 'undefined' &&
+    new URLSearchParams(window.location.search).has('gp')
+  ) {
+    return true;
+  }
+  return (import.meta.env as Record<string, string | undefined>).VITE_GP_BUILD === '1';
+}