fix(tapbacks): a failure is said even if Blip closed while it ran

[ianswope]

Follow-up to the "not addressed" note on #138.

Cause

The imsg-react Process lived in the BlipView that started it. The popout's surface and the app window are both destroyed on close, and on a macOS 27 gateway a run takes 15-75 s. Close Blip mid-run and the Process went with the view, so its exit, and the reason on stderr, never reached the status line.

Fix

• The run moves to BarWidget (runTapback, reactProc), which outlives both surfaces. One run at a time, as before.
• Every exit emits tapbackExited(chat, ok). On success a view on that chat settles its pending pill exactly as before.
• A failure is held in tapbackFailures[chat] until a view showing that conversation takes it (takeTapbackFailure): immediately if it is on screen, otherwise when the conversation is next opened, in the popout or the window. This replaces the view-local tapbackNote. A new tapback drops an older failure for the same chat.
• The not-started guard (sawExit) moves with the Process.
• imsg-react: on macOS 27 the newest-message check now runs before Messages is touched, so a tapback on an older message is refused in about a second without bringing Messages forward or opening the conversation.

Verified

• On a 27.0.1 gateway, a tapback on an older message showed tapback: on this macOS only the newest message in a conversation can take a tapback in the status line; react.log: not-last exit=65.
• Deployed with a shell restart; no plugin errors in the qs log; the popout renders.
• tapbacks.test.ts asserts the new wiring, including that BlipView no longer owns reactProc. test_react.py 60 pass (the not-last refusal now also asserts the conversation was never opened). bun test 750 pass.

[nixfred]

Merged, thank you Ian, and for closing your own loose end from #138 the same night. Moving the run into the bar widget is the right owner: a 15-75 s run on macOS 27 was always going to outlive a closed window, and a failure that waits for its conversation beats one that vanishes. Refusing an older message before Messages comes forward is a nice touch too. 750 tests and every bridge test green; live on my Mac and both Linux machines.