fix(auth): end the session when a refetch of the account says 401

A failed refetch keeps the last good payload in the query cache, so a
session that expired while the app was open still read as signed in:
the shell kept its pages, and every save failed with "sign in again".
A 401 or 403 now ends the session even with a cached account; a network
failure or a 5xx still holds the last known state.

src/hooks/__tests__/use-auth-offline-gate.test.ts

@@ -136,6 +136,30 @@ describe("useAuth — last-known state on a non-verdict failure", () => {
   });
 });
 
+describe("useAuth — a verdict on a refetch", () => {
+  it("ends the session on a 401 although the cache still holds the account", () => {
+    store.set("healthlog-was-authenticated", "1");
+    queryState.data = { id: "u1", username: "someone" };
+    queryState.isError = true;
+    queryState.error = new ApiError("unauthorized", 401);
+
+    const auth = useAuth();
+    expect(auth.isAuthenticated).toBe(false);
+    expect(auth.user).toBeNull();
+    expect(auth.isAuthUnknown).toBe(false);
+  });
+
+  it("keeps the cached account through a network failure on a refetch", () => {
+    queryState.data = { id: "u1", username: "someone" };
+    queryState.isError = true;
+    queryState.error = new TypeError("Failed to fetch");
+
+    const auth = useAuth();
+    expect(auth.isAuthenticated).toBe(true);
+    expect(auth.user).toEqual({ id: "u1", username: "someone" });
+  });
+});
+
 describe("session-end marker lifecycle", () => {
   it("clearCachesForSessionEnd drops the marker so the next cold offline launch lands on the login gate", () => {
     store.set("healthlog-was-authenticated", "1");

src/hooks/use-auth.ts

@@ -536,11 +536,14 @@ export function useAuth() {
   // /auth/login and wiped every offline cache (data 7→0 entries, pages 1→0
   // in the audit measurement), making the whole offline story unreachable.
   const verdictUnknown = query.isError && isAuthVerdictUnknown(query.error);
-  const isAuthenticated =
-    !!query.data || (verdictUnknown && readWasAuthenticated());
+  // A 401/403 on a refetch ends the session although the cache still holds
+  // the last good payload: React Query keeps `data` when a refetch fails.
+  const sessionEnded = query.isError && !verdictUnknown;
+  const user = sessionEnded ? null : (query.data ?? null);
+  const isAuthenticated = !!user || (verdictUnknown && readWasAuthenticated());
 
   return {
-    user: query.data ?? null,
+    user,
     isLoading: query.isLoading,
     isAuthenticated,
     /**
@@ -550,7 +553,7 @@ export function useAuth() {
      * wipe: a wipe without a 401/403 verdict would destroy the offline
      * caches on every network blip.
      */
-    isAuthUnknown: !query.data && verdictUnknown,
+    isAuthUnknown: !user && verdictUnknown,
     error: query.error,
     refetch: query.refetch,
   };