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

backend/app/api/deps.py

@@ -47,6 +47,12 @@ def user_from_token(session: Session, token: str) -> User | None:
 
 
 def get_current_user(session: SessionDep, token: TokenDep) -> User:
+    """Resolve the bearer token to its user.
+
+    Every failure here is an authentication failure, so all of them answer 401:
+    a token naming a user who no longer exists is a session to log in again,
+    not a missing resource to report.
+    """
     try:
         payload = jwt.decode(
             token, settings.SECRET_KEY, algorithms=[security.ALGORITHM]
@@ -54,14 +60,15 @@ def get_current_user(session: SessionDep, token: TokenDep) -> User:
         token_data = TokenPayload(**payload)
     except (InvalidTokenError, ValidationError):
         raise HTTPException(
-            status_code=status.HTTP_403_FORBIDDEN,
+            status_code=status.HTTP_401_UNAUTHORIZED,
             detail="Could not validate credentials",
         )
     user = session.get(User, token_data.sub)
-    if not user:
-        raise HTTPException(status_code=404, detail="User not found")
-    if not user.is_active:
-        raise HTTPException(status_code=400, detail="Inactive user")
+    if user is None or not user.is_active:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="This session is no longer valid — please log in again",
+        )
     return user