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/api/schema.ts

@@ -1169,6 +1169,7 @@ export interface components {
         LoginRequest: {
             username: string;
             password: string;
+            return_session: null | boolean;
         };
         LoginResponse: {
             user_id: string;
@@ -1176,6 +1177,7 @@ export interface components {
             csrf_token: string;
             /** Format: date-time */
             expires_at: unknown;
+            session_token: null | string;
         };
         MeDto: {
             kind: string;