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/Flow/EdgeInspector.tsx

@@ -1,3 +1,5 @@
+import { ArrowRight } from "lucide-react"
+
 import { Button } from "@/components/ui/button"
 import { Popover, PopoverAnchor, PopoverContent } from "@/components/ui/popover"
 import { ScrollArea } from "@/components/ui/scroll-area"
@@ -15,13 +17,15 @@ function relativeTime(ts: number | null | undefined): string {
 
 export type InspectedEdge = {
   message: string
+  /** Node titles, so the popover names the two ends in the user's own words. */
+  from: string
+  to: string
   x: number
   y: number
 }
 
 /**
- * What last travelled along an edge. The value is whatever the producing node
- * published, shown as it was serialised.
+ * What last travelled along an edge, and between which two nodes.
  */
 export function EdgeInspector({
   edge,
@@ -48,7 +52,16 @@ export function EdgeInspector({
         className="w-72 p-3"
         data-testid="edge-inspector"
       >
-        <p className="font-mono text-xs text-muted-foreground">
+        <p className="flex items-center gap-1.5 text-sm">
+          <span className="min-w-0 flex-1 truncate font-medium">
+            {edge.from}
+          </span>
+          <ArrowRight className="size-3.5 shrink-0 text-muted-foreground" />
+          <span className="min-w-0 flex-1 truncate text-right font-medium">
+            {edge.to}
+          </span>
+        </p>
+        <p className="mt-1 font-mono text-xs text-muted-foreground">
           {displayName(flow, edge.message)}
         </p>