fix(desktop): resolve login shell when loading env (#26449)

Co-authored-by: Brendan Allan <14191578+Brendonovich@users.noreply.github.com>

packages/desktop/src/main/shell-env.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test"
 
-import { isNushell, mergeShellEnv, parseShellEnv } from "./shell-env"
+import { isNushell, mergeShellEnv, parseShellEnv, resolveUserShell } from "./shell-env"
 
 describe("shell env", () => {
   test("parseShellEnv supports null-delimited pairs", () => {
@@ -34,6 +34,13 @@ describe("shell env", () => {
     expect(env.OPENCODE_CLIENT).toBe("desktop")
   })
 
+  test("resolveUserShell falls back to the login shell before /bin/sh", () => {
+    expect(resolveUserShell("/custom/env-shell", "/bin/zsh")).toBe("/custom/env-shell")
+    expect(resolveUserShell(undefined, "/bin/zsh")).toBe("/bin/zsh")
+    expect(resolveUserShell(undefined, "unknown")).toBe("/bin/sh")
+    expect(resolveUserShell(undefined, undefined)).toBe("/bin/sh")
+  })
+
   test("isNushell handles path and binary name", () => {
     expect(isNushell("nu")).toBe(true)
     expect(isNushell("/opt/homebrew/bin/nu")).toBe(true)