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/MenuBar.swift

@@ -205,7 +205,8 @@ struct SessionSummary {
         switch issue {
         case .cliMissing: "Installing the container runtime…"
         case .cliOutdated: "Updating the container runtime…"
-        case .serviceStopped, .kernelMissing: "Starting the container runtime…"
+        case .serviceStopped: "Starting the container runtime…"
+        case .kernelMissing: "Downloading the Linux kernel…"
         default: "Building the VM…"
         }
     }