Add offline installation support with repo folder and updated dependencies

Features:
- Create repo/ folder structure for offline packages
  - python-wheels/ for Python packages (.whl files)
  - system-packages/ for APT packages and .deb files
- Add download_offline_packages.sh to populate Python wheels
- Add download_deb_packages.sh to download system .deb packages
- Update install.sh with smart online/offline detection
  - Auto-detects repo folder and uses offline packages
  - Falls back to online installation if repo is empty
  - Supports --offline flag for explicit offline mode
- Update requirements.txt with async dependencies:
  - aiohttp==3.9.1 for async HTTP client
  - asyncio==3.4.3 for async I/O framework
- Add OFFLINE_INSTALLATION.md with complete guide
- Add .gitignore to exclude downloaded packages from repo
- Document all system dependencies in apt-packages.txt

Downloaded packages:
- 18 Python wheels with all dependencies
- Total size: ~50 MB for Python packages
- Ready for offline deployment

repo/system-packages/.gitkeep

@@ -0,0 +1 @@
+# Keep this directory in git

repo/system-packages/apt-packages.txt

@@ -0,0 +1,39 @@
+# System packages required for Kivy Signage Player
+# Install with: sudo apt install -y $(cat apt-packages.txt)
+
+# Python and build tools
+python3-pip
+python3-setuptools
+python3-dev
+
+# SDL2 libraries for Kivy
+libsdl2-dev
+libsdl2-image-dev
+libsdl2-mixer-dev
+libsdl2-ttf-dev
+
+# Multimedia libraries
+libportmidi-dev
+libswscale-dev
+libavformat-dev
+libavcodec-dev
+
+# Compression
+zlib1g-dev
+
+# Video playback
+ffmpeg
+libavcodec-extra
+
+# Audio support
+gstreamer1.0-plugins-base
+gstreamer1.0-plugins-good
+gstreamer1.0-plugins-bad
+gstreamer1.0-alsa
+
+# Network tools
+wget
+curl
+
+# Additional video codecs
+libx264-dev