The install overlay had no way to stop a running install — the runner already
supported an abortSignal, but nothing drove it. Wire it end to end:
- main.cjs holds an AbortController for the active runBootstrap and aborts it
on a new hermes:bootstrap:cancel IPC and on app quit, so quitting/cancelling
mid-install actually kills install.sh/ps1 instead of orphaning it.
- runBootstrap bails before spawning anything if the signal is already aborted.
- Install overlay gains a "Cancel install" button while a bootstrap is active;
a cancel surfaces the recovery overlay (retry/repair).
Test: electron/bootstrap-runner.test.cjs asserts the already-aborted early
return (no spawn) via `node --test`.
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