[Bug] Session timeout silently terminates /ultraplan while UI remains active, losing unsaved feedback

Bug Description
Title: /ultraplan browser UI accepts feedback into an already-terminated session, then loses it on submit

What happened:
I ran /ultraplan on a non-trivial planning task. The session terminated server-side after 90 minutes with "no
approval." because I took my dog for a walk. I didn't see that in the browser — the feedback composer remained active and editable. I spent ~20 minutes writing detailed feedback on the plan. When I hit submit, the API returned:

{
"error": {
"message": "Cannot send events to inactive session: session_01DPsH1geoW9EQfYnRFvHtxq",
"reason": "",
"type": "invalid_request_error"
},
"request_id": "req_011Ca4gHW6jnkm4gjvSrQ29n",
"type": "error"
}

The error had no recovery path and my 20 minutes of feedback were lost.

Expected:
Any of these would have prevented the data loss:

1. The browser should disable the composer (or show a clear banner) as soon as the session goes inactive, so I

can't waste time writing into a dead session.

1. Feedback drafts should be persisted client-side (localStorage) so a failed submit — for any reason —

doesn't destroy the work.

1. On submit failure, the UI should keep the composer contents and offer retry / copy-to-clipboard, not just

surface a raw API error.

1. User activity in the composer should extend the session timeout, or the timeout should be much longer than

90 minutes given the "async" framing.

Actual:

• Session terminated silently in the background while the UI still looked live.
• Composer accepted 20 minutes of typing into a session the server already considered dead.
• Submit returned invalid_request_error with no retry, no draft preservation, no export.
• All feedback lost, plus the tokens spent generating the plan.

Impact:
The core value prop of /ultraplan is async inline feedback on a plan. A 90-minute wall clock doesn't match
"async" — real review cycles (walk away, come back, think) routinely exceed that. And …
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