What

• Updated the quick-reference table to include "desktop" in the Platform.OS row and add Platform.is_desktop to the booleans row, with a short note that "desktop" means the pn preview Tkinter backend.

Why

• The quick-reference table in docs/api/platform.md (around line 19) says Platform.OS is "ios", "android", or "test", and only lists is_ios / is_android / is_test. Since the desktop preview backend landed, the code also reports "desktop" and exposes Platform.is_desktop (see src/pythonnative/platform.py, lines 39, 77-78, 83, and 94).

Testing

• No tests, only doc change

Risks/Impact

• No risks, only doc change

Docs/Follow-ups

• Docs updated

Closes #39