Make this app a PWA

I mentioned in #122 that implementing a service worker could help alleviate bugs due to connection issues and extended sessions. If this is done, we wouldn't be too far away from having this app be installable as a PWA. To see what this would look like, see my project nextcloud-notes-webview. We could just pull the CSS from there to get the mobile UI working nicely (haven't seen any bugs yet!)

I am not too well-versed in PWAs, but is this possible? With the file manipulation involved in this app, I assume we would have to have some type of "sync" between the cached files on an offline device and the stored notes. This is where my knowledge breaks down (and this may be where it gets way too complicated and not worth it at all).

Why do it in the first place? I think a consistent experience across devices is important when choosing a note-taking app. It has its limits this way (long notes >400 lines are very slow on mobile) but if we can do it, I think it will make new user's transitions easier and help any other current users out there that feel the same way I do.

If that's not in the cards, I would love to get my chunk of scss into this repo. It would only affect apps with the custom useragent suffix I set for the webview (notes-android), so it wouldn't change anything for anyone, but then I could package my app and have it work instantly for anyone who wants to use it :)

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Start by reading the related discussion in #122 and reviewing the linked nextcloud-notes-webview project, especially notes-android.scss. Clarify whether the goal is a full PWA with offline synchronization or only the Android-specific styling; done would require an agreed scope and validation of the resulting installable or styled experience.