Bug: composer 4xx/5xx clears typed text - must retain on error

[heavygee]

Problem

When the message composer submits a message and the hub responds with a 4xx or 5xx, the composer clears the input -- the operator's typed text is gone. This is destructive on errors, which is exactly when the operator most needs their input retained so they can retry or save it.

Example: composer fires POST /api/sessions/:id/messages, hub returns 503 during a restart -> text discarded.

Proposal

Don't clear the composer on error. Only clear on a successful 2xx.

Recommended approach:

1. Optimistically render the message as "sending" but keep a copy of the input bound.
2. On 2xx -- clear input, finalise the optimistic render.
3. On 4xx / 5xx / network error -- restore the input (don't ever destroy it), surface the error inline, allow retry.

Acceptance

• Submit -> 500/502/503/network error -> composer text is not cleared
• Submit -> 400/401/403 -> composer text is not cleared, error surfaces inline
• Submit -> 2xx -> composer clears as today
• Operator can edit the retained text and retry without re-typing

Notes

• Pair with UI: hover popup on session refs with 2-line summary + quick-reply input #8 -- hub restarts (the 0.52s blip) are the most common cause of mid-send errors.
• Look at web/src/components/Composer*.tsx (or wherever the message composer lives) for onSubmit + setState reset order.
• Add a small visual on the input border to indicate "send failed -- press to retry."

[heavygee]

PR opened upstream: tiann#773