feat: add AppImage and Flatpak packaging support

- Add AppImage build using linuxdeploy
- Add Flatpak build using flatpak-builder
- Create AppRun script for .NET runtime sharing
- Create Flatpak manifest and desktop file
- Integrate both formats into release workflow
- Upload .AppImage and .flatpak as release artifacts

This provides comprehensive Linux packaging support:
- DEB: For Debian/Ubuntu systems
- Linglong: For Deepin/UOS systems
- AppImage: Universal Linux format (download and run)
- Flatpak: Universal Linux format (sandboxed)

All formats share the same .NET runtime for efficiency.

packaging/linux/AppImage/AppRun

@@ -0,0 +1,22 @@
+#!/bin/bash
+# AppImage entry point for LanMountainDesktop
+
+SELF=$(readlink -f "$0")
+HERE="${SELF%/*}"
+
+# Find the app directory with .NET runtime
+APP_DIR=""
+for dir in "$HERE/usr/bin"/app-*; do
+  if [ -d "$dir" ] && [ -f "$dir/LanMountainDesktop.dll" ]; then
+    APP_DIR="$dir"
+    break
+  fi
+done
+
+if [ -n "$APP_DIR" ]; then
+  export DOTNET_ROOT="$APP_DIR"
+  export PATH="$DOTNET_ROOT:$PATH"
+fi
+
+# Launch the application
+exec "$HERE/usr/bin/LanMountainDesktop.Launcher" "$@"