fix(man_pages): correct broken Windows guard (closes #1898)

[Niranjan-png]

Fixes broken Windows guard using sys.platform. Regression test added.

[Niranjan-png]

Note on CI failures (verified facts only):

• 5 failing tests: test_auth.py (2), test_cli_ui.py (1), test_encoding.py (2 — big5 charset detection).
• My PR changes only: httpie/output/ui/man_pages.py (+2 -1) and new tests/test_issue_1898_regression.py.
• The failing tests do not involve the changed files; they cover auth, CLI UI, and encoding areas unrelated to the fix.
• I attempted to verify these failures on the base branch locally, but reproduction was blocked because pytest_httpbin is missing from the environment; I am not claiming conclusively that they fail on the base.
• I have not modified PR fix(man_pages): correct broken Windows guard (closes #1898) #1967; please confirm whether these are known/pre-existing CI failures.