feat: warn about legacy tariff settings

frontend/src/styles/admin.css

@@ -1948,6 +1948,54 @@
   flex: 0 0 auto;
 }
 
+.admin-settings-warning {
+  display: grid;
+  grid-template-columns: auto minmax(0, 1fr) auto;
+  gap: 12px;
+  align-items: center;
+  min-width: 0;
+  margin: 14px 18px 4px;
+  padding: 12px 14px;
+  border: 1px solid var(--warning-border);
+  border-radius: 10px;
+  background: color-mix(in srgb, var(--warning) 10%, var(--admin-surface));
+  color: var(--admin-text);
+}
+
+.admin-settings-warning > svg {
+  color: var(--warning);
+}
+
+.admin-settings-warning-copy {
+  display: grid;
+  gap: 3px;
+  min-width: 0;
+}
+
+.admin-settings-warning-copy strong {
+  font-size: 13px;
+  line-height: 1.3;
+}
+
+.admin-settings-warning-copy p {
+  margin: 0;
+  color: var(--admin-muted);
+  font-size: 12px;
+  line-height: 1.45;
+}
+
+.admin-settings-warning-link {
+  color: var(--accent);
+  font-size: 12px;
+  font-weight: 700;
+  text-decoration: none;
+  white-space: nowrap;
+}
+
+.admin-settings-warning-link:hover {
+  text-decoration: underline;
+}
+
 .admin-setting:last-child {
   border-bottom: 0;
 }
@@ -2704,6 +2752,16 @@
     align-self: flex-start;
   }
 
+  .admin-settings-warning {
+    grid-template-columns: auto minmax(0, 1fr);
+    margin: 12px 14px 4px;
+  }
+
+  .admin-settings-warning-link {
+    grid-column: 2;
+    justify-self: start;
+  }
+
   .admin-card-head {
     padding: 12px 14px;
   }