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/routes/_layout.tsx

@@ -25,7 +25,9 @@ function Layout() {
     <SidebarProvider className="bg-card">
       <AppSidebar />
       <SidebarInset className="bg-card">
-        <header className="sticky top-0 z-10 flex h-16 shrink-0 items-center gap-2 border-b px-4">
+        {/* The sidebar carries its own collapse control; a phone has no
+            sidebar on screen to carry it. */}
+        <header className="sticky top-0 z-10 flex h-16 shrink-0 items-center gap-2 px-4 md:hidden">
           <SidebarTrigger className="-ml-1 text-muted-foreground" />
         </header>
         <main className="flex-1 p-6 md:p-8">