fix(frontend): hide viewer-only risk-compute UI, proactively renew session

- Executive Dashboard no longer auto-triggers POST /risk/compute or shows
  the Refresh scores button for roles the backend rejects (viewer), which
  was causing a 403 on load
- AuthContext now proactively renews the session cookie every 20 minutes
  while the app is open, so an actively-used session doesn't rely solely
  on the reactive on-401 refresh racing against its own token's expiry

frontend/src/api/auth.ts

@@ -35,6 +35,11 @@ export async function getMe(): Promise<User> {
   return data;
 }
 
+/** Silently renew the session cookie. Throws if there is nothing to renew. */
+export async function refreshToken(): Promise<void> {
+  await client.post("/auth/refresh");
+}
+
 /** Change the current user's password. */
 export async function changePassword(
   currentPassword: string,

frontend/src/context/AuthContext.tsx

@@ -10,6 +10,7 @@ import {
   login as apiLogin,
   logout as apiLogout,
   getMe,
+  refreshToken as apiRefreshToken,
 } from "../api/auth";
 import type { User } from "../types/models";
 import ChangePasswordModal from "../components/ChangePasswordModal";
@@ -45,6 +46,21 @@ export function AuthProvider({ children }: { children: ReactNode }) {
     refreshUser().finally(() => setIsLoading(false));
   }, [refreshUser]);
 
+  // Proactively renew the session cookie while the app is open, so an
+  // actively-used session never actually reaches the hard 8-hour expiry
+  // (relying solely on the reactive on-401 refresh risks losing the race
+  // against the token's own expiry — see api/client.ts).
+  useEffect(() => {
+    if (!user) return;
+    const PROACTIVE_REFRESH_INTERVAL_MS = 20 * 60 * 1000;
+    const id = setInterval(() => {
+      apiRefreshToken().catch(() => {
+        // Reactive on-401 handling in api/client.ts takes over from here.
+      });
+    }, PROACTIVE_REFRESH_INTERVAL_MS);
+    return () => clearInterval(id);
+  }, [user]);
+
   const login = useCallback(async (username: string, password: string) => {
     await apiLogin(username, password);
     const me = await getMe();

frontend/src/pages/ExecutiveDashboardPage.tsx

@@ -32,6 +32,11 @@ import { getCoverageByTactic } from "../api/metrics";
 import { getThreatActors } from "../api/threat-actors";
 import { getTechniques, type TechniqueSummary } from "../api/techniques";
 import { getRiskProfiles, computeRiskScores, type RiskProfile } from "../api/risk";
+import { useAuth } from "../context/AuthContext";
+
+// Roles allowed to trigger a risk-score recompute (matches the backend's
+// POST /risk/compute role gate) — viewer must never call this endpoint.
+const CAN_COMPUTE_RISK_ROLES = ["admin", "red_lead", "blue_lead"];
 
 // ── Score Gauge Component ────────────────────────────────────────────
 
@@ -159,6 +164,8 @@ function sinceDate(days: number | null): string | undefined {
 
 export default function ExecutiveDashboardPage() {
   const navigate = useNavigate();
+  const { user } = useAuth();
+  const canComputeRisk = CAN_COMPUTE_RISK_ROLES.includes(user?.role ?? "");
 
   const [timeRange, setTimeRange] = useState<TimeRange>("all");
   const rangeOption = TIME_RANGE_OPTIONS.find((o) => o.value === timeRange)!;
@@ -214,9 +221,10 @@ export default function ExecutiveDashboardPage() {
     },
   });
 
-  // Auto-compute on first load if no profiles exist
+  // Auto-compute on first load if no profiles exist — only for roles the
+  // backend actually allows to call POST /risk/compute (viewer would 403).
   useEffect(() => {
-    if (!loadingRisk && riskProfiles !== undefined && riskProfiles.length === 0) {
+    if (canComputeRisk && !loadingRisk && riskProfiles !== undefined && riskProfiles.length === 0) {
       computeMutation.mutate();
     }
   // eslint-disable-next-line react-hooks/exhaustive-deps
@@ -708,6 +716,7 @@ export default function ExecutiveDashboardPage() {
             Critical Gaps — Top 10 by Risk Priority
             <MetricTooltip title="Critical Gaps" description="The 10 most dangerous attack techniques with no detection coverage, ranked by a composite risk score based on: how many threat actors use this technique, recent threat intelligence, and test history." context="These are your highest-priority gaps — real adversaries actively use these techniques and you cannot currently detect them." />
           </h2>
+          {canComputeRisk && (
             <button
               onClick={() => computeMutation.mutate()}
               disabled={computeMutation.isPending}
@@ -721,6 +730,7 @@ export default function ExecutiveDashboardPage() {
               )}
               {computeMutation.isPending ? "Computing…" : "Refresh scores"}
             </button>
+          )}
         </div>
         <div className="overflow-x-auto">
           <table className="w-full text-sm">