Fix #95 win32 copy/paste from/to system clipboard

[clach04]

• Read/write the Windows clipboard via the Win32 API (CF_UNICODETEXT), replacing clip.exe (unicode-unsafe)
• Paste (ctrl+v, ctrl+shift+v, shift+insert) prefers the OS clipboard, falling back to Textual's internal buffer; image paste unchanged
• Selection/copy mirrors to the OS clipboard alongside Textual's buffer
• TODO notes: Linux clipboard reading unimplemented; mouse capture left on (right-click paste under CMD.exe does not work)
• Document keyboard/clipboard behavior in README and welcome screen

[clach04]

GHA failure appears to be due to GHA environment:

Run uv run python -m pyright .
/home/runner/work/kon/kon/src/kon/ui/clipboard.py
  /home/runner/work/kon/kon/src/kon/ui/clipboard.py:56:21 - error: "WinDLL" is not a known attribute of module "ctypes" (reportAttributeAccessIssue)
  /home/runner/work/kon/kon/src/kon/ui/clipboard.py:57:23 - error: "WinDLL" is not a known attribute of module "ctypes" (reportAttributeAccessIssue)
2 errors, 0 warnings, 0 informations
WARNING: there is a new pyright version available (v1.1.408 -> v1.1.411).
Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`

Error: Process completed with exit code 1.

[0xku]

Thanks for all the PRs. Will review them tomorrow.