Show a queued chat message as pending, not as sent

[oldsj-mainloop]

Summary

Follow-up to #258. That PR's own report claimed it added a small "queued" badge/state on the message, but its diff never touched a message bubble at all — it only stopped the composer from locking while a turn is in flight (Composer.tsx, ThreadBody.tsx). A message sent while a turn is open — which the server durably queues and 202s as status: "queued" — rendered in the transcript exactly like an already-delivered message, because deliveryNotices() only ever produced a notice for a failed or uncertain delivery row, never for one the server already reports as queued.

The server already carries everything needed to distinguish this: NativeDeliveryInfo.state includes queued in the delivery ledger (api/openapi.yaml), and GET /main-thread / GET /sessions/{id}/native already return that row live (confirmed by reading the server code — this is not a client-only guess). No server change was needed.

• web/src/lib/delivery.ts: deliveryNotices() now also emits a notice for a queued delivery (label Queued, not retryable — nothing has been attempted yet).
• web/src/features/chat/DeliveryNotice.tsx: a distinct muted style and explanatory text for the queued notice, separate from the failed (red) and unconfirmed (yellow) ones.
• web/src/features/chat/MessageBubble.tsx: the bubble itself is dimmed while queued (not just the notice below it), so it reads as pending at a glance; the dimming and notice both clear automatically once the server reports the delivery promoted off queued — no client-side timer.
• Composer stays non-blocking during a turn, unchanged from Stop the chat composer locking while a turn is in flight #258.
• Updated docs/specs/chat.md and the message/delivery-receipt design docs to describe the new per-message Queued state (distinct from the thread-level status words, which are unaffected).

Test plan

• web/src/lib/delivery.test.ts: new unit tests for the queued notice and its clearing once promoted.
• web/src/features/chat/MessageBubble.test.tsx: queued bubble renders dimmed with a Queued notice and no Retry; reverts to normal once the notice is gone.
• web/src/features/chat/ChatPage.test.tsx: end-to-end repro — a message sent while a turn is in flight shows Queued and a dimmed bubble, and both clear once the backend promotes the delivery row. Verified this test (and the two unit ones above) fail against the pre-fix code, reproducing the reported bug, and pass after the fix.
• Full suite: tsc -b, eslint ., vitest run all clean (one unrelated flaky test in hitl-card.test.tsx failed once in the full run and passed in isolation — not touched by this change).
• Synced to mainloop-dev at this head: healthy, migrations ok, smoke passed.

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