feat(ui): wallet storefront redesign — split buy/spend, compact balance, exchange confirm

Reworks the Wallet screen to be more compact and to separate the two flows, and fixes a
reported bug.

Bug: buying a value with too few chips showed the generic "Something went wrong" toast — the
backend's `insufficient_chips` (409) code was propagated correctly but had no i18n mapping, so
`errorKey` fell back to `error.generic`. Add `error.insufficient_chips` / `error.product_not_found`.
Also disable a value's Exchange button up front when the spendable balance cannot cover it (the
server stays authoritative).

Redesign:
- a compact **balance** row leads the screen — the context's own chips (🪙 N) then any linked
  other-platform chips behind that platform's logo (monospaced digits); inside a VK/TG store only
  that store's own segment shows;
- the benefits section is renamed **Active**, moved above the store, shows one inline line
  (hints + ad-free) and hides when nothing is active;
- the **store** splits by a two-way toggle into **Buy chips** (money packs + the watch-an-ad row at
  the top) and **Spend chips** (values, exchanged for chips);
- a value's action reads **Exchange** and opens a single confirmation dialog (chip spends are
  instant, with no provider window to confirm them); the existing cross-platform store-compliance
  warning folds into that same dialog when the spend would draw VK/TG chips.

No payments-model or wire change; the money→chips→values model is unchanged (verified: money buys
only chip packs, values are chips-only — "no-ads for money" is structurally impossible).

Tests: wallet e2e updated for the new layout (chromium + webkit green). Docs: FUNCTIONAL (+_ru)
wallet story rewritten. App bundle budget 126 → 127 KB (always-loaded settings hub).

ui/src/lib/i18n/en.ts

@@ -228,20 +228,19 @@ export const en = {
   // --- wallet ---
   'wallet.title': 'Wallet',
   'wallet.tab': 'Wallet',
-  'wallet.balance': 'Balance',
-  'wallet.noChips': 'No chips yet',
   'wallet.source.vk': 'VK',
   'wallet.source.telegram': 'Telegram',
   'wallet.source.direct': 'Web',
-  'wallet.viewOnly': 'view only',
-  'wallet.benefits': 'Benefits',
+  'wallet.active': 'Active',
+  'wallet.activeHints': 'Hints: {n}',
   'wallet.noAdsUntil': 'No ads until {date}',
   'wallet.noAdsForever': 'No ads forever',
-  'wallet.adsOn': 'Ads are on',
-  'wallet.hints': 'Hints {n}',
   'wallet.store': 'Store',
+  'wallet.tabBuy': 'Buy chips',
+  'wallet.tabSpend': 'Spend chips',
   'wallet.empty': 'The store is empty for now',
   'wallet.buy': 'Buy',
+  'wallet.spend': 'Exchange',
   'wallet.rewardWatch': 'Watch an ad for {n} 🪙',
   'wallet.rewardCta': 'Watch',
   'wallet.rewardCredited': '+{n} 🪙 for watching',
@@ -256,10 +255,11 @@ export const en = {
   'wallet.cur.RUB': '₽',
   'wallet.cur.VOTE': 'votes',
   'wallet.cur.XTR': '⭐',
-  'wallet.warnTitle': 'Web-only purchase',
+  'wallet.spendTitle': 'Confirm exchange',
+  'wallet.spendBody': 'Exchange {n} 🪙 for “{title}”?',
+  'wallet.spendConfirm': 'Exchange',
   'wallet.warnBody':
     'This spends your VK/Telegram chips, and the benefit will work on the web and in the app only — because of store rules.',
-  'wallet.warnConfirm': 'Continue',
   'wallet.warnCancel': 'Cancel',
   'about.title': 'About',
   'about.tab': 'Info',
@@ -313,6 +313,8 @@ export const en = {
   'error.invalid_email': 'Enter a valid email address.',
   'error.invalid_config': 'Invalid game settings.',
   'error.not_found': 'Not found.',
+  'error.insufficient_chips': 'Not enough chips.',
+  'error.product_not_found': 'This item is no longer available.',
   'error.session_invalid': 'Your session expired. Please sign in again.',
   'error.unauthenticated': 'Please sign in.',
   'error.rate_limited': 'Too many requests, slow down.',