Add Settings Screen

[apbendi]

Add a new tab for the settings screen, with the following functionality:

• [ ] Ability to set the base URL of Kasparov API endpoint, defaulting to local KDX for now
  • Should trigger a cache wipe and re-fetch of all wallet data
• [ ] Ability to export encrypted wallet mnemonic file
• [ ] Ability to display seed
  • Taking either of these two actions should satisfy the backup requirement and dismiss the banner warning, if not already done