Add French Windows installation guide

[Copilot]

French-speaking users requested Windows installation documentation in their native language.

Changes

• Added INSTALLATION_WINDOWS_FR.md (651 lines)

  • System requirements (minimal/recommended configs)
  • 4 installation methods: Windows executable (GitHub Actions/Release/local build), direct Python, virtual environment, pip install
  • Windows-specific commands for cmd/PowerShell
  • Installation verification steps
  • Troubleshooting section covering 10+ common Windows issues (Python PATH, Visual C++ Redistributable, GPU/CUDA, MediaPipe, webcam detection, etc.)
  • Cross-references to existing French docs (BUILD_EXE_GUIDE_FR.md, COMMENT_OBTENIR_EXE.md)

• Updated README.md

  • Added French guide link in Installation section (line 45)
  • Added reference in Support section (line 1461)

Navigation

French users see this immediately in the Installation section:

## 🚀 Installation

> 🇫🇷 **Pour les utilisateurs francophones** : [Guide d'Installation Windows en Français](INSTALLATION_WINDOWS_FR.md)
> 
> 🇫🇷 **For French speakers** : [Windows Installation Guide in French](INSTALLATION_WINDOWS_FR.md)

Original prompt

explique comment installer ceci sous windows

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