refactor(frontend): finish design-system migration — chatbot, a11y, lint

- Rewrite chatbot container + panel to new surface/signal/ink tokens
  (was still on dead glass/text-primary tokens -> wrong colors)
- loading-skeleton: glass -> surface-2
- Fix a11y: SVG charts role=img+aria-label, audio aria-label,
  message-entry as real <button>, tooltip biome-ignore (intentional)
- Type messages/page initialPage (noImplicitAny)
- tsc clean, next build green, biome 0 errors

services/frontend/src/components/charts/radial-gauge.tsx

@@ -43,6 +43,8 @@ export function RadialGauge({
         height={size}
         viewBox={`0 0 ${size} ${size}`}
         className="-rotate-90"
+        role="img"
+        aria-label={`${Math.round(pct * 100)}% ${label ?? "gauge"}`}
       >
         <circle
           cx={size / 2}

services/frontend/src/components/charts/sparkline.tsx

@@ -21,7 +21,15 @@ export function Sparkline({
 }: SparklineProps) {
   const id = useId().replace(/:/g, "");
   if (data.length < 2)
-    return <svg width={width} height={height} className={className} />;
+    return (
+      <svg
+        width={width}
+        height={height}
+        className={className}
+        role="img"
+        aria-label="No data"
+      />
+    );
 
   const min = Math.min(...data);
   const max = Math.max(...data);
@@ -46,6 +54,8 @@ export function Sparkline({
       viewBox={`0 0 ${width} ${height}`}
       className={className}
       preserveAspectRatio="none"
+      role="img"
+      aria-label="Trend sparkline"
     >
       <defs>
         <linearGradient id={`spark-${id}`} x1="0" y1="0" x2="0" y2="1">