Implement a basic app shell

[RobinTTY]

As a first step towards a functional client I want to create a shell for the application that provides:

• A navigation which can be used to switch between different views (e.g. transactions, accounts, reports, etc.)
• Light/Dark Mode switch
• Access to settings and the user account (if there is going to be one)

Related tasks:

• #49
• #50
• #51