Fix OS reporting in /system_stats API to use sys.platform

Replace os.name with sys.platform for more detailed OS identification.
This change provides better OS differentiation:
- Windows: "nt" -> "win32"
- macOS: "posix" -> "darwin"
- Linux: "posix" -> "linux"

Previously, both macOS and Linux returned "posix", making them
indistinguishable. Now each OS has a unique identifier, aligning
with the Registry Specifications for proper compatibility checks.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server.py

@@ -558,7 +558,7 @@ class PromptServer():
 
             system_stats = {
                 "system": {
-                    "os": os.name,
+                    "os": sys.platform,
                     "ram_total": ram_total,
                     "ram_free": ram_free,
                     "comfyui_version": __version__,