导航加载失败时把原因显示出来，并支持点击重试

之前 /api/navigation 失败是静默的：整条导航直接空掉，只能翻控制台才知道
是 401、跨域还是网络层错误。现在失败时在导航位置显示一枚琥珀色徽标
「导航加载失败（HTTP 401 / Network Error …）· 点此重试」，点一下重新拉取。
成功时清空错误态；仍保留上次成功结果的缓存兜底。

web/src/components/top-nav.tsx

@@ -224,6 +224,10 @@ export function TopNav({ branding = DEFAULT_BRANDING }: { branding?: Branding })
   const [isLoadingApiKey, setIsLoadingApiKey] = useState(false);
   const [profile, setProfile] = useState<MyProfile | null>(null);
   const [customNavigation, setCustomNavigation] = useState<NavigationItem[]>([]);
+  // 导航加载失败时把真实原因摆到界面上：之前失败是静默的，整条导航直接空掉，
+  // 只能靠翻控制台才知道是 401、跨域还是网络层错误。
+  const [navError, setNavError] = useState<string | null>(null);
+  const [navReloadToken, setNavReloadToken] = useState(0);
   const [openCustomNavigationId, setOpenCustomNavigationId] = useState("");
   const [isProfileOpen, setIsProfileOpen] = useState(false);
   const [nicknameValue, setNicknameValue] = useState("");
@@ -352,7 +356,10 @@ export function TopNav({ branding = DEFAULT_BRANDING }: { branding?: Branding })
         try {
           const data = await fetchNavigation();
           const items = data.items || [];
-          if (active) setCustomNavigation(items);
+          if (active) {
+            setCustomNavigation(items);
+            setNavError(null);
+          }
           writeCachedNavigation(items);
           return;
         } catch (error) {
@@ -360,6 +367,7 @@ export function TopNav({ branding = DEFAULT_BRANDING }: { branding?: Branding })
           const status = (error as { status?: number }).status;
           if (status || attempt === 2) {
             setCustomNavigation(readCachedNavigation());
+            setNavError(status ? `HTTP ${status}` : (error as Error).message || "网络错误");
             return;
           }
           await new Promise((resolve) => window.setTimeout(resolve, 400 * (attempt + 1)));
@@ -370,7 +378,7 @@ export function TopNav({ branding = DEFAULT_BRANDING }: { branding?: Branding })
     return () => {
       active = false;
     };
-  }, [session?.role, sessionKey]);
+  }, [session?.role, sessionKey, navReloadToken]);
 
   useEffect(() => {
     if (!sessionKey) {
@@ -624,6 +632,19 @@ export function TopNav({ branding = DEFAULT_BRANDING }: { branding?: Branding })
                 opacity: activeIndicator.ready ? 1 : 0,
               }}
             />
+            {customNavigation.length === 0 && navError ? (
+              <button
+                type="button"
+                onClick={() => {
+                  setNavError(null);
+                  setNavReloadToken((value) => value + 1);
+                }}
+                title={navError}
+                className="relative z-10 inline-flex h-9 shrink-0 items-center gap-1.5 whitespace-nowrap rounded-xl bg-amber-50 px-3 text-[13px] font-semibold text-amber-700 outline-none ring-1 ring-amber-200 transition-colors hover:bg-amber-100 focus-visible:ring-2 focus-visible:ring-amber-400"
+              >
+                导航加载失败（{navError}）· 点此重试
+              </button>
+            ) : null}
             {customNavigation.map((item) => {
               const itemKey = `custom:${item.id}`;
               const active = activeCustomNavigationKey === itemKey;