Recover from a dead session, and report a node problem once

An expired or orphaned session left the app on a half-rendered page instead of
the login screen: a properly signed token naming a user who no longer exists
answered 404, which the client does not treat as an authentication failure. All
failures in get_current_user are 401 now, and the client stops retrying them,
so a dead session goes straight back to the login screen.

- Clicking an edge names the two nodes it runs between, not just the message.
- A node with a problem shows one badge carrying the whole explanation, rather
  than a corner badge and a status dot saying the same thing twice. The dot is
  back to what it is good at: whether the node ran.
- The sidebar's collapse control sits in the sidebar, where a phone still finds
  one in the chrome because there is no sidebar on screen to hold it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016WzrvW7rjQbynnhF6pxh6i

frontend/src/components/Sidebar/AppSidebar.tsx

@@ -7,6 +7,7 @@ import {
   SidebarContent,
   SidebarFooter,
   SidebarHeader,
+  SidebarTrigger,
 } from "@/components/ui/sidebar"
 import useAuth from "@/hooks/useAuth"
 import { type Item, Main } from "./Main"
@@ -33,7 +34,15 @@ export function AppSidebar() {
       className="[&>[data-sidebar=sidebar]]:bg-card/80 [&>[data-sidebar=sidebar]]:backdrop-blur-md [&>[data-sidebar=sidebar]]:shadow-e2"
     >
       <SidebarHeader className="px-4 py-6 group-data-[collapsible=icon]:px-0 group-data-[collapsible=icon]:items-center">
-        <Logo variant="responsive" />
+        <div className="flex w-full items-center justify-between gap-2 group-data-[collapsible=icon]:justify-center">
+          {/* Collapsed, the rail has room for one thing, and that is the way
+              back out. */}
+          <span className="group-data-[collapsible=icon]:hidden">
+            <Logo variant="responsive" />
+          </span>
+          {/* On a phone the sidebar is a sheet with its own way in and out. */}
+          <SidebarTrigger className="hidden shrink-0 text-muted-foreground md:inline-flex" />
+        </div>
       </SidebarHeader>
       <SidebarContent>
         <Main items={items} />