Show billing estimate and clarify session replacement

src/styles/pages/legacy-pages.css

@@ -11915,6 +11915,21 @@
   gap: 6px;
 }
 
+.wb-composer__billing-estimate {
+  max-width: 138px;
+  padding: 6px 9px;
+  border: 2px solid #111;
+  background: #fffbe8;
+  color: #111;
+  font-size: 12px;
+  font-weight: 800;
+  line-height: 1;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  box-shadow: 2px 2px 0 #111;
+}
+
 .wb-composer__send-primary {
   display: inline-flex;
   align-items: center;

src/styles/themes/dark-green.css

@@ -1794,6 +1794,14 @@
   box-shadow: none;
 }
 
+.web-shell[data-ui-theme="dark-green"] .wb-composer__billing-estimate {
+  border: 1px solid var(--border-default);
+  border-radius: 999px;
+  background: var(--bg-elevated);
+  color: var(--fg-body);
+  box-shadow: none;
+}
+
 .web-shell[data-ui-theme="dark-green"] .wb-composer__send-primary:hover:not(:disabled) {
   background: var(--accent-hover);
   color: var(--dg-button-text);