Consolidate state management

## Why?

In-memory state is held in a variety of stores following different patterns. This makes it hard to reason about the code, complicates changes and prevents multi-account support.

## What?

We’ll devise a unified pattern for storing state and migrate all existing stores to the new system.

## Plan

```[tasklist]
### Tasks
- [ ] Convert remaining stores (probably need subtask for each one)
- [ ] Remove `SdkContextClass.instance`
```

## Internal references

myhours: https://app.myhours.com/#/projects/2028403/overview