[Request Invoicing] Refactor payment state logic to use State Machine. Improve error handling

[MantisClone]

Problem

The Invoice Dashboard: Invoice View and Single Invoice components contain messy state logic with many booleans. This makes the state logic very difficult to reason about and maintain. There is a strong suspicion that it doesn't handle errors or deviations from the happy path in the approval and payment workflow.

Proposed Solution

Refactor the state logic to use a State Machine - where all states are defined in a single enumeration. This will simplify reasoning and maintenance - it makes it obvious what code will need to update when adding new states or changing existing ones.

[MantisClone]

I propose we use a state machine to fix the payment flow logic.