FR: Add PWA capabilities

[semohr]

We could easily add some pwa features. What I feel like could be a nice feature would be:

• Notifications for new inbox entries see Push notifications for new tags and imported folders #148
• Proper fullscreen on android and ios devices

Should be easy to support without changeing much things. Am not sure tho if this needs https, might make it difficult to setup.

Maybe we can collect some more feature ideas here.

Ref: https://web.dev/learn/pwa/