fix(decoration): strip 4 accent / status-hue leaks across hero surfaces

DESIGN.md 'Tunable-but-Scarce Rule' caps accent at <=10% of any
screen and reserves the accent + status hues for action / status,
never decoration. Four sites were spending those tokens on pure
chrome:

- Subscription.tsx purchased-traffic bar: zone.mainHex linear-gradient
  on a non-status progress bar; the per-purchase bar inherited the
  user's GLOBAL traffic-zone color (a fresh purchase could read
  'critical' just because the user's overall usage was). Replaced
  with solid accent-500 fill — same affordance, honest semantics.

- SubscriptionCardActive tariff badge: zone-colored linear-gradient
  background + zone-colored label. The tariff name has no traffic-zone
  semantics, so tinting it by the global traffic zone was a
  Status-Hue Lockout violation. Switched to glass innerBg/innerBorder.

- Balance hero card: bg-gradient-to-br from-accent-500/10 + glow prop
  on Card. Removed; flat surface, the giant numeric is the affordance.
  Eliminates the SaaS hero-metric template tell.

- Login background: two stacked fixed inset gradients (linear +
  accent radial halo) read as the airdrop / crypto aesthetic
  PRODUCT.md explicitly anti-references. Replaced with the plain
  body bg-dark-950.

src/pages/Balance.tsx

@@ -210,9 +210,12 @@ export default function Balance() {
         <h1 className="text-2xl font-bold text-dark-50 sm:text-3xl">{t('balance.title')}</h1>
       </motion.div>
 
-      {/* Balance Card */}
+      {/* Balance Card — flat surface; the giant numeric carries the
+          weight. The previous accent gradient + glow leaked accent into
+          decoration (DESIGN.md Tunable-but-Scarce Rule) and read as the
+          SaaS hero-metric template. */}
       <motion.div variants={staggerItem}>
-        <Card className="bg-gradient-to-br from-accent-500/10 to-transparent" glow>
+        <Card>
           <div className="mb-2 text-sm text-dark-400">{t('balance.currentBalance')}</div>
           <div className="text-4xl font-bold text-dark-50 sm:text-5xl">
             {formatAmount(balanceData?.balance_rubles || 0)}