A turn that ends without a clean finish now shows a neutral marker, so an
interrupted answer is visible instead of trailing off silently. Errors keep
their existing red banner; this covers the aborted case.
- chat-stopped-notice.tsx: new neutral (gray) notice component
- chat-thread.tsx: live marker driven by useChat onFinish flags — distinguishes
a manual Stop (isAbort) from a dropped connection (isDisconnect); cleared when
the next turn streams; flushNext still runs only on a clean finish
- message-item.tsx: per-message marker in reopened history for finishReason
'aborted' with no error (combined wording, since the server can't tell a
manual Stop from a dropped connection)
- ai-chat.types.ts: add metadata.finishReason; rowToUiMessage now carries it
- en-US: three new strings
Frontend only — the server already persists partial work and finishReason and
replays it to the model on the next turn (continue, not restart).
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