Handle coagulate links

[LGro]

Guides
iOS: https://docs.flutter.dev/cookbook/navigation/set-up-universal-links
Android: https://docs.flutter.dev/cookbook/navigation/set-up-app-links

Tasks

• wellknown file for coagulate.social
• add router to app
• update xcode project as necessary including the right capabilities
• updatie the android app manifest accordingly
• host assetlinks.json on coagulate.social
• figure our how to handle the fragment, consider using https://pub.dev/packages/uni_links and see https://docs.flutter.dev/ui/navigation/deep-linking
• handle malformed links gracefully with a helpful error message (currently showing "GoException: no routes for location" for /c on iOS and for malformed /en/c#123 goes to qr code scanning mode)