Fix first-run setup timing out on the kernel download

On a Mac without the Linux kernel, setup ran `container system start
--enable-kernel-install` with a 120 s limit. That command downloads a
697 MB archive without printing anything, so most connections hit the
limit and the window showed `timedOut([...])`.

- Start the service with --disable-kernel-install (10 min: a first
  start still pulls the 69 MB base filesystem).
- Download the kernel in its own step with progress, then hand the file
  to `container system kernel set --tar ... --digest ...`, which checks
  it against the runtime's digest.
- Opening the setup window no longer starts the download; Set Up does.
- Timeouts and runner errors read as sentences.
- A rebuild deletes the image it replaced, which frees about 2 GB each
  time, and deletes or stops the builder VM unless it was already
  running (it kept about 2.8 GB of memory).
- Setup asks for 10 GB free, measured on a real setup.

Sources/Knotchy/AppDelegate.swift

@@ -158,6 +158,7 @@ final class AppDelegate: NSObject, NSApplicationDelegate {
         quitting = true
         setupFlow.cancel()
         if setupFlow.isDownloading { SystemRuntimeInstaller.removeLeftovers() }
+        if controller.state == .settingUp(.kernelMissing) { ContainerCLIBackend.removeKernelDownloads() }
         // Not Now mid-build closes the window before the build ends.
         if setupFlow.allRequiredDone { UserDefaults.standard.set(true, forKey: SetupLaunchRule.completedKey) }
         // Synchronous on purpose: terminate: often runs inside a non-reentrant main-queue block; awaiting here deadlocked.