feat(web): add backend switcher and connection status

Split the login page into backend list and credentials, show connection status in the sidebar and top bar, and render dialogs in a portal.

web/shared/theme/tokens.css

@@ -59,6 +59,10 @@
 :root[data-theme="dark"] {
   color-scheme: dark;
 
+  /* 暗色下的阴影和遮罩要用纯黑，不然和深色背景差不多，看不出层次 */
+  --app-shadow: #000000;
+  --app-scrim: #000000;
+
   --app-bg: #181714;
   --app-card: #211f1b;
   --app-card-inset: #2b2924;