fix(desktop): resolve login shell when loading env

[qwq202]

Issue for this PR

Closes #26356

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Desktop can start with SHELL unset, especially from GUI launch contexts. In that case getUserShell() fell back to /bin/sh, so the existing shell env probe did not read the user login shell where Homebrew, mise, nvm, and similar PATH setup often live.

This changes the fallback to use os.userInfo().shell before /bin/sh, keeping process.env.SHELL as the explicit override. That lets the existing -il shell env loader probe the actual login shell and pass the resulting environment to the sidecar and local MCP processes.

This should fully address the Desktop-specific spawn npx ENOENT class of failures for local MCP servers that work in the CLI but fail from the macOS app because the app did not inherit the user's login-shell PATH.

How did you verify your code works?

• bun test src/main/shell-env.test.ts from packages/desktop
• bun run oxlint packages/desktop/src/main/shell-env.ts packages/desktop/src/main/shell-env.test.ts
• Pre-push bun turbo typecheck passed
• Manual Desktop-path verification: launched the patched Electron main process with SHELL unset and PATH=/usr/bin:/bin:/usr/sbin:/sbin, simulating a macOS GUI launch without Homebrew on PATH. The patched app logged [server] Loaded shell environment with -il (...), proving it resolved the real login shell instead of falling back to /bin/sh.
• Manual MCP verification with a ReactBits config using command: ["npx", "-y", "reactbits-dev-mcp-server"]: after the patched Desktop process loaded the login-shell env, the failure no longer reached spawn npx ENOENT.
• The remaining ReactBits log was MCP error -32601: Method not found failed to get prompts, which happens after local MCP startup and is not the local process spawn/PATH failure fixed here.

Screenshots / recordings

N/A - no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR