- shutil.which("python3") → use "python" on Windows (no python3.exe)
- Extract _find_base_python() helper shared by both installer paths:
  - Unix: login-shell probe via bash/zsh (guarded by sys.platform check)
  - Windows: conda/system paths use python.exe under AppData and C:\
  - Removes ~50 lines of duplicated detection logic
- Error hint text is now platform-aware (Windows path vs Unix path)