Platform modules

[Hanziness]

The app will potentially run inside a Flutter app, too (#246), requiring a different approach to notification handling and such. The app should have a runtime configuration variable that allows it to compile for both the web (where it will be a PWA) and a web app embedded into a mobile app (where it doesn't need PWA functionality and the notifications are handled by the app framework).

Tasks

• [ ] Allow specifying the platform to compile for (web or mobile)
• [ ] (mobile) Only compile /timer for the mobile platform
• [ ] (mobile) Create the mobile platform module composable that forwards app events to the parent app
• [ ] (mobile) Allow setting other parameters (like additional padding requested)
• [ ] (mobile) Play audio files from the containing app (or set them as notification sounds)
• [ ] (mobile) Make the app self-contained -- ie. work without internet
• [ ] (web) Only inject PWA scripts and manifest link on web