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/display/src/App.tsx

@@ -21,7 +21,7 @@ export function App() {
             </NavLink>
             <a href="/admin/">管理</a>
           </nav>
-          <SourceButton />
+          <SourceButton variant="status" />
           <ThemeToggle />
         </div>
       </header>