Reapply "feat(desktop): terminal pane (#5081)"

This reverts commit 03a0312e5b.

packages/util/src/shell.ts

@@ -0,0 +1,13 @@
+export function shell() {
+  const s = process.env.SHELL
+  if (s) return s
+  if (process.platform === "darwin") {
+    return "/bin/zsh"
+  }
+  if (process.platform === "win32") {
+    return process.env.COMSPEC || "cmd.exe"
+  }
+  const bash = Bun.which("bash")
+  if (bash) return bash
+  return "bash"
+}