server2-preview: FATAL on Windows — tmux socket dir permission check always sees 0777 on NTFS

## Summary

`server2-preview-20260831-080203-baeff37da` (and at least `server2-preview-20260831-023000-e53ff25f0`) cannot start on Windows. The new tmux socket directory permission check requires 0700, but Go's `os.Stat` on NTFS always reports 0777, so the check always fails.

## Steps to reproduce

1. Windows 11 (NTFS), `quicktui-server-windows-amd64.exe` from `server2-preview-20260831-080203-baeff37da`
2. tmux-windows 3.6a installed and in PATH
3. Run `quicktui-server serve --tmux-socket C:\Users\\AppData\Local\QuickTUI\tmux-socket\default`

## Actual result

```
[FATAL] [SERVER] tmux socket dir "C:\Users\\AppData\Local\QuickTUI\tmux-socket"
has permissions 0777, want 0700 (owner rwx, no group/other); refusing tmux backend.
```

Even after restricting the directory with `icacls /inheritance:r /grant ":(OI)(CI)F"`, Go still reports 0777 because NTFS does not support POSIX permission bits.

## Expected result

The server should start normally on Windows. The 0700 check is meaningful on Linux/macOS but is a false positive on NTFS.

## Additional notes

- Tried setting `QUICKTUI_SKIP_PERM_CHECK=1` env var — not recognized.
- Also hit `QUICKTUI_TMUX_SOCKET must be an absolute path: "\tmp\tmux--1\default"` when the server auto-detects the socket path on Windows (missing drive letter).
- Stable release `20260809-09` works fine on the same machine (no tmux socket dir permission check).

## Environment

- Windows 11 Home China 10.0.26200 (AMD64)
- Go runtime reports `os.FileMode` 0777 for all NTFS directories
- tmux-windows 3.6a-win32 (arndawg/tmux-windows via winget)

## Suggestion

Skip or relax the POSIX permission check on `runtime.GOOS == "windows"`, or use Windows ACL APIs (`GetNamedSecurityInfo` / `GetEffectiveRightsFromAcl`) to verify owner-only access on NTFS.

Start at the Go implementation of the tmux socket directory permission check used by `quicktui-server serve`, then reproduce the behavior on Windows NTFS. The fix is done when the server starts on Windows without treating NTFS’s 0777 mode as a failure, while the 0700 check remains meaningful on POSIX systems; also verify the reported Windows auto-detected socket path issue.

Written by the indexing model from the issue text.