fix: route_ready rejects 5xx responses; remove spinner from wait page

route_ready returned True on 502 Bad Gateway, causing premature redirect
before the container was actually serving. Now only accepts < 500 responses.
Spinner removed from wait page per user request.

app/runtime.py

@@ -532,7 +532,7 @@ def wait_for_session_route(session_id: str, timeout_seconds: int = 6) -> bool:
                 allow_redirects=False,
                 timeout=1.5,
             )
-            if resp.status_code != 404:
+            if resp.status_code < 500:
                 return True
         except Exception:
             pass
@@ -554,7 +554,7 @@ def route_ready(path: str) -> bool:
                 timeout=1.5,
                 verify=verify,
             )
-            if resp.status_code != 404:
+            if resp.status_code < 500:
                 return True
         except Exception:
             continue