fix: createOpencode function failure on Windows due to spawn usage

[webuilder]

Adds { shell: process.platform === 'win32' } when spawning child processes on Windows, allowing the system to resolve opencode.cmd from the PATH.

Issue for this PR

Closes #18792

Type of change

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

What does this PR do?

When I use opencode sdk on windows. I called createOpencode function, then got a fail

node:internal/child_process:286
      const err = new ErrnoException(exitCode, syscall);
                  ^

Error: spawn opencode ENOENT
    at ChildProcess._handle.onexit (node:internal/child_process:286:19)
    at onErrorNT (node:internal/child_process:484:16)
    at process.processTicksAndRejections (node:internal/process/task_queues:89:21) {
  errno: -4058,
  code: 'ENOENT',
  syscall: 'spawn opencode',
  path: 'opencode',
  spawnargs: [ 'serve', '--hostname=127.0.0.1', '--port=0' ]
}

So I changed server.ts, I added a shell flag to the spawn function arguments, setting it to true for Windows and false otherwise.

How did you verify your code works?

Verified the fix on Windows 11 and Ubuntu.

Screenshots / recordings

No ui changed.

Checklist

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

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Hey! Your PR title Fix createOpencode function failure on Windows due to spawn usage doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
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• test: or test(scope): adding or updating tests

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[github-actions]

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