TUI: stop button in the composer while a turn is running

[plombeer31]

Problem

While the agent is working there is no visible way to stop it. Esc, Ctrl+C and /abort all abort the turn, but every one of them is keyboard lore — nothing on screen tells the operator a running turn can be stopped, or where. And once stopped, the chat renders an error wall (Turn failed [cancelled]: This operation was aborted — or, when the abort races the stream, even Turn failed [model]: model returned empty content) for something the operator did on purpose.

Change

1. A clickable stop chip inside the input field, next to send →, rendered only while status === "running":

• ComposerStopButton — a sibling of ComposerSendButton with the same MouseTarget pattern and overlay layer. No disabled state: it only exists when there is a run to stop.
• Drawn on the palette's error ground with a measured ink (readableOn), legible across all eleven themes.
• PromptShell gains running / onStop props; tui-app wires onStop to exactly the pair of calls the Esc branch makes (onAbort() + abort_requested).

2. A stopped turn says who stopped it, and offers a retry. The reducer treats loop_failed as stopped-by-user when the category is cancelled or an abort_requested is on the books (state.aborting — every abort entry point sets it, finishRun clears it), which also catches the abort/stream race that used to dress the user's own stop as a provider failure. Instead of the warn wall it leaves a calm system notice — Agent stopped by user. — carrying the aborted turn's prompt as retryText, and chat-log hangs the existing [try again] beside [copy] on exactly that notice, resending the user's prompt: a mistaken stop is one click to undo.

Keyboard behaviour is unchanged; both pieces are additive.

Tests

• App-level mouse test through real Ink layout and hit-testing: chip absent while idle, appears when a submit starts a turn, click lands on onAbort exactly once, chip leaves when the run ends.
• Reducer tests: cancelled loop_failed → calm notice + retryText + cancelled outcome; [model] failure during a requested abort → still stopped-by-user; the same failure with no abort on the books keeps the warn styling; no retryText when there is no prompt to re-run.
• Chat-log render test: [try again] hangs under the stopped notice and only there.

tsc --noEmit clean; components + reducer + key-binding/submit suites: 675 tests green. Verified live in the TUI against a real provider (stop mid-turn → Agent stopped by user. + [copy] [try again]).