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/Login.tsx

@@ -321,9 +321,10 @@ export default function Login() {
           safeBottom > 0 ? `${safeBottom + 16}px` : 'calc(1rem + env(safe-area-inset-bottom, 0px))',
       }}
     >
-      {/* Background gradient */}
-      <div className="fixed inset-0 bg-gradient-to-br from-dark-950 via-dark-900 to-dark-950" />
-      <div className="fixed inset-0 bg-[radial-gradient(ellipse_at_top,_var(--tw-gradient-stops))] from-accent-500/10 via-transparent to-transparent" />
+      {/* Flat background — the previous two layered gradients (linear
+          + accent radial halo) read as the airdrop / crypto aesthetic
+          PRODUCT.md explicitly anti-references. Body bg-dark-950 carries
+          the surface alone. */}
 
       {/* Language switcher */}
       <div