handle windows char encoding errors

[dillonroach]

• serve.py:9-13 - reconfigure sys.stdout/sys.stderr to UTF-8 with errors='replace' on Windows.
• serve.py:117-119 - Popen in run_with_pipes now decodes child output as UTF-8 explicitly with errors='replace', instead of inheriting the locale codec (cp1252).
• serve.py:161-162 - pass PYTHONIOENCODING=utf-8 and PYTHONUTF8=1 to subprocesses, so any spawned Python (e.g. hf CLI) emits UTF-8 instead of the legacy code page.
• app.py:13-17 - same Windows stdout/stderr reconfigure for safety.

The root cause was the hf CLI emitting ✓ (U+2713) status glyphs; subprocess.Popen(text=True, ...) without an explicit encoding= defaulted to Windows' cp1252, and re-encoding via print() then raised the charmap error. Forcing UTF-8 on both sides fixes it without changing behavior on Linux/macOS.