fix(tokens): add missing showCalc state — page was crashing on render

The Tokens screen referenced showCalc / setShowCalc in the Cost calculator
button and modal but never declared the state hook, so the component
threw ReferenceError on mount and rendered blank.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

services/web-ui/public/screens-admin.jsx

@@ -498,6 +498,7 @@ function GroupsPanel({ groups, users, onChange }) {
 function Tokens() {
   const [burned, setBurned] = React.useState(14340);
   const [rate, setRate] = React.useState(2.4);
+  const [showCalc, setShowCalc] = React.useState(false);
 
   React.useEffect(() => {
     const i = setInterval(() => {