Feat/ping devices from dashboard v1.2.0

[herzhenr]

Closes #1

• online state of all devices is shown on the home page. The devices get pinged periodically in the background but a user can also swipe down to refresh manually
• Default values for the 2s timeout and the 12s ping interval are hardcoded in lib/constants.dart for now.
• dart SDK is upgraded to v3.0.0 to allow for new language features (e.g. Records)
• interacting with lib/services/database.dart was reworked. When an action (delete, add or change a device) is made the devices aren't loaded from disk every time first but are loaded from ram and directly written to disk