add subscription tiers

frontend/src/components/TierPicker.css

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+.tier-picker {
+  display: grid;
+  grid-template-columns: repeat(3, 1fr);
+  gap: 1rem;
+}
+
+.tier-picker__card {
+  position: relative;
+  display: flex;
+  flex-direction: column;
+  padding: 1.5rem 1.25rem;
+  background-color: var(--color-bg-elevated);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-xl);
+  text-align: center;
+}
+
+.tier-picker__card--highlighted {
+  border-color: var(--color-primary);
+  box-shadow: 0 0 0 1px var(--color-primary);
+}
+
+.tier-picker__card--selected {
+  border-color: var(--color-success);
+  box-shadow: 0 0 0 1px var(--color-success);
+}
+
+.tier-picker__badge {
+  position: absolute;
+  top: -10px;
+  left: 50%;
+  transform: translateX(-50%);
+  background-color: var(--color-primary);
+  color: white;
+  font-size: 0.7rem;
+  font-weight: 700;
+  padding: 2px 12px;
+  border-radius: 10px;
+  white-space: nowrap;
+}
+
+.tier-picker__name {
+  margin-bottom: 0.5rem;
+  font-size: 1.05rem;
+  color: var(--color-text);
+}
+
+.tier-picker__price {
+  margin-bottom: 1.25rem;
+}
+
+.tier-picker__amount {
+  font-size: 2rem;
+  font-weight: 700;
+  color: white;
+}
+
+.tier-picker__period {
+  font-size: 0.85rem;
+  color: var(--color-text-dimmed);
+}
+
+.tier-picker__features {
+  list-style: none;
+  padding: 0;
+  margin: 0 0 1.25rem;
+  text-align: left;
+  flex-grow: 1;
+}
+
+.tier-picker__feature {
+  padding: 0.3rem 0;
+  font-size: 0.82rem;
+  color: var(--color-text-label);
+}
+
+.tier-picker__feature--disabled {
+  color: var(--color-text-dimmed);
+  opacity: 0.55;
+}
+
+.tier-picker__check {
+  color: var(--color-primary);
+  font-weight: bold;
+  margin-right: 0.4rem;
+}
+
+.tier-picker__dash {
+  margin-right: 0.4rem;
+}
+
+.tier-picker__btn {
+  width: 100%;
+  padding: 0.6rem;
+  background-color: var(--color-bg-card);
+  border: 1px solid var(--color-border);
+  color: var(--color-text);
+  cursor: pointer;
+  border-radius: var(--radius-md);
+  font-weight: 600;
+}
+
+.tier-picker__btn:hover {
+  background-color: var(--color-primary);
+  border-color: var(--color-primary);
+}
+
+.tier-picker__btn--selected {
+  background-color: var(--color-success);
+  border-color: var(--color-success);
+  color: white;
+}
+
+@media (max-width: 768px) {
+  .tier-picker {
+    grid-template-columns: 1fr;
+    max-width: 400px;
+    margin: 0 auto;
+  }
+}

frontend/src/components/TierPicker.jsx

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+import "./TierPicker.css";
+
+const TIERS = [
+  {
+    id: "free",
+    name: "Free Trial Access",
+    price: "$0",
+    period: "",
+    features: [
+      "Monitor 1 blockchain address",
+      "Daily email digest alert",
+      "1 transaction alert type per monitored address",
+    ],
+    disabledFeatures: [
+      "Real-time Discord alerts",
+      "Real-time Telegram alerts",
+      "Real-time Slack alerts",
+    ],
+  },
+  {
+    id: "premium",
+    name: "Premium Access",
+    price: "$1.99",
+    period: "/month",
+    features: [
+      "Monitor 3 blockchain addresses",
+      "Daily email digest alert",
+      "Real-time Discord alerts",
+      "Real-time Telegram alerts",
+      "2 transaction alert types per monitored address",
+    ],
+    disabledFeatures: [
+      "Real-time Slack alerts",
+    ],
+    highlighted: true,
+  },
+  {
+    id: "pro",
+    name: "Pro Access",
+    price: "$11.99",
+    period: "/month",
+    features: [
+      "Monitor unlimited blockchain addresses",
+      "Daily email digest alert",
+      "Real-time Discord alerts",
+      "Real-time Telegram alerts",
+      "Real-time Slack alerts",
+      "Unlimited transaction alert types per monitored address",
+    ],
+    disabledFeatures: [],
+  },
+];
+
+export default function TierPicker({ onSelect, selectedTier }) {
+  return (
+    <div className="tier-picker">
+      {TIERS.map((tier) => (
+        <div
+          key={tier.id}
+          className={`tier-picker__card${tier.highlighted ? " tier-picker__card--highlighted" : ""}${selectedTier === tier.id ? " tier-picker__card--selected" : ""}`}
+        >
+          {tier.highlighted && (
+            <div className="tier-picker__badge">Most Popular</div>
+          )}
+          <h3 className="tier-picker__name">{tier.name}</h3>
+          <div className="tier-picker__price">
+            <span className="tier-picker__amount">{tier.price}</span>
+            {tier.period && (
+              <span className="tier-picker__period">{tier.period}</span>
+            )}
+          </div>
+          <ul className="tier-picker__features">
+            {tier.features.map((f) => (
+              <li key={f} className="tier-picker__feature">
+                <span className="tier-picker__check">&#10003;</span> {f}
+              </li>
+            ))}
+            {tier.disabledFeatures.map((f) => (
+              <li key={f} className="tier-picker__feature tier-picker__feature--disabled">
+                <span className="tier-picker__dash">&mdash;</span> {f}
+              </li>
+            ))}
+          </ul>
+          <button
+            className={`btn tier-picker__btn${selectedTier === tier.id ? " tier-picker__btn--selected" : ""}`}
+            onClick={() => onSelect(tier.id)}
+          >
+            {selectedTier === tier.id ? "Selected" : "Select"}
+          </button>
+        </div>
+      ))}
+    </div>
+  );
+}

frontend/src/components/UpgradeBanner.css

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+.upgrade-banner {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 1rem;
+  padding: 0.75rem 1rem;
+  margin-bottom: 1rem;
+  border-radius: var(--radius-md);
+  background-color: #3a2e00;
+  border: 1px solid #aa7700;
+  color: #ffcc44;
+}
+
+.upgrade-banner__message {
+  font-size: 0.88rem;
+}
+
+.upgrade-banner__link {
+  background: none;
+  border: 1px solid #aa7700;
+  color: #ffcc44;
+  padding: 0.25rem 0.75rem;
+  border-radius: var(--radius-md);
+  cursor: pointer;
+  font-size: 0.82rem;
+  font-weight: 600;
+  white-space: nowrap;
+}
+
+.upgrade-banner__link:hover {
+  background-color: #4a3800;
+}

frontend/src/components/UpgradeBanner.jsx

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+import { useNavigate } from "react-router-dom";
+import "./UpgradeBanner.css";
+
+export default function UpgradeBanner({ message, linkTo = "/account" }) {
+  const navigate = useNavigate();
+
+  return (
+    <div className="upgrade-banner">
+      <span className="upgrade-banner__message">{message}</span>
+      <button
+        className="upgrade-banner__link"
+        onClick={() => navigate(linkTo)}
+      >
+        Upgrade
+      </button>
+    </div>
+  );
+}