mission canvas: cap description height with own scroll

Long refined descriptions (Opus tends to emit full section spines)
pushed the tabs + toolbar past the viewport with no way to reach
them. Cap the description block at 38vh with its own overflow-y so
the header stays reachable no matter how long the brief gets.

frontend/src/components/dashboard/MissionCanvas.tsx

@@ -404,7 +404,14 @@ export function MissionCanvas({
         </div>
         <h1 style={{ margin: 0, fontSize: 20, color: "#f3f3f5" }}>{mission.title}</h1>
         {mission.description && (
-          <div style={{ marginTop: 4 }}>
+          <div
+            style={{
+              marginTop: 4,
+              maxHeight: "38vh",
+              overflowY: "auto",
+              paddingRight: 8,
+            }}
+          >
             <MarkdownBlock source={mission.description} />
           </div>
         )}