fix: 提升管理端启动与导航稳定性

- 等待初始路由就绪并提供可恢复的启动失败页面

- 收敛失效登录导航并按需加载 SVG 图标资源

easyflow-ui-admin/app/src/startup-error.css

@@ -0,0 +1,52 @@
+.startup-error {
+  display: grid;
+  place-items: center;
+  min-height: 100vh;
+  padding: 32px;
+  color: CanvasText;
+  background: Canvas;
+}
+
+.startup-error__content {
+  width: min(100%, 480px);
+  text-align: center;
+}
+
+.startup-error__title {
+  margin: 0;
+  font-size: 24px;
+  font-weight: 600;
+  line-height: 1.4;
+}
+
+.startup-error__description {
+  margin: 16px 0 24px;
+  line-height: 1.6;
+  color: GrayText;
+}
+
+.startup-error__retry {
+  min-width: 112px;
+  min-height: 40px;
+  padding: 0 24px;
+  font: inherit;
+  line-height: 1.6;
+  color: HighlightText;
+  cursor: pointer;
+  background: Highlight;
+  border: 0;
+  border-radius: var(--radius, 8px);
+}
+
+.startup-error__retry:hover {
+  filter: brightness(0.96);
+}
+
+.startup-error__retry:active {
+  filter: brightness(0.9);
+}
+
+.startup-error__retry:focus-visible {
+  outline: 2px solid Highlight;
+  outline-offset: 3px;
+}