feat: support AppImage builds

- add CI builds for AppImage builds
- add AppImage metadata

Signed-off-by: TheOneWithTheBraid <the-one@with-the-braid.cf>

appimage/.gitignore

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+FluffyChat.AppDir
+*.AppImage

appimage/AppRun

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+#!/bin/sh
+
+cd "$(dirname "$0")"
+exec ./fluffychat

appimage/FluffyChat.desktop

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+[Desktop Entry]
+Type=Application
+Version=1.0
+Name=FluffyChat
+Comment=Matrix Client. Chat with your friends
+Exec=AppRun
+Icon=fluffychat
+Terminal=false
+Categories=Network;Chat;InstantMessaging;X-Matrix;

appimage/README.md

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+# FluffyChat AppImage
+
+FluffyChat is provided as AppImage too. To Download, visit fluffychat.im.
+
+## Building
+
+- Ensure you install `appimagetool`
+
+```shell
+flutter build linux
+
+# copy binaries to appimage dir
+cp -r build/linux/{x64,arm64}/release/bundle appimage/FluffyChat.AppDir
+cd appimage
+
+# prepare AppImage files
+cp FluffyChat.desktop FluffyChat.AppDir/
+mkdir -p FluffyChat.AppDir/usr/share/icons
+cp ../assets/logo.png FluffyChat.AppDir/usr/share/icons/fluffychat.png
+
+# build the AppImage
+appimagetool FluffyChat.AppDir
+```