feat(ads): VK post-move interstitial + retire deprecated hint_balance/paid_account

Interstitial video after a confirmed play or a hint, VK-only, offline
banner-only. The gate is client-mirrored: the backend puts the config
cooldowns (global 5m / vs_ai 30m / hint 1m) and a suppressed flag (the
no-ads / no_banner gate, same as the banner) on Profile.ads via adsFor;
the client self-gates on a per-kind last-shown time in localStorage, with
the VITE_ADS_STUB contour "ad fired" toast. Never fires after a pass,
exchange or resign. maybeShowInterstitial + vkShowInterstitial; the codec
and gateway transcode carry the ads block.

Also retires the deprecated accounts.hint_balance / paid_account domain
usage (expand-contract, code only — the columns stay for a later DROP so
image rollback stays DB-safe): drop the Account fields and their scan, the
dead account.SpendHint, account.GrantHints and the admin grant-hints
action; the in-game hint display now comes wholly from the payments hint
benefit. Banner eligibility and account merge no longer read the legacy
flags.

Tests: ads.test.ts (the client-mirrored gate), the codec ads round-trip,
the gateway ads transcode test, and a profile ads-config integration test
(cooldowns + suppressed under no-ads / no_banner). Docs: PAYMENTS §10 (+ru)
interstitial, the decision amends, backend README, the plan.

ui/src/lib/codec.ts

@@ -14,6 +14,7 @@ import type {
   RobotBlockEntry,
   RobotFriendRequestEntry,
   Banner,
+  AdsConfig,
   BannerCampaign,
   BestMove,
   BestMoveTile,
@@ -461,6 +462,7 @@ export function decodeProfile(buf: Uint8Array): Profile {
     notificationsInAppOnly: p.notificationsInAppOnly(),
     variantPreferences: decodeVariantPreferences(p),
     banner: decodeBanner(p),
+    ads: decodeAds(p),
     email: s(p.email()),
     telegramLinked: p.telegramLinked(),
     vkLinked: p.vkLinked(),
@@ -497,6 +499,19 @@ function bannerTriple(bg: string | null, fg: string | null, link: string | null)
   return bg && fg && link ? { bg, fg, link } : null;
 }
 
+// decodeAds projects the optional post-move interstitial config of a Profile, or undefined when
+// absent (an old backend); the client then shows no interstitial.
+function decodeAds(p: fb.Profile): AdsConfig | undefined {
+  const a = p.ads();
+  if (!a) return undefined;
+  return {
+    cooldownGlobalS: a.cooldownGlobalS(),
+    cooldownVsAiS: a.cooldownVsAiS(),
+    cooldownHintS: a.cooldownHintS(),
+    suppressed: a.suppressed(),
+  };
+}
+
 // decodeBanner projects the optional advertising-banner block of a Profile, or
 // undefined when the viewer is not eligible (the field is absent).
 function decodeBanner(p: fb.Profile): Banner | undefined {