[Feature Request]: Unify MPM-initiated transaction initiation

### Preflight Checklist

- [x] I have read the [Contributing Guidelines](https://github.com/EnAccess/micropowermanager/blob/main/CONTRIBUTING.md) for this project, if it exists.
- [x] I agree to follow the [Code of Conduct](https://github.com/EnAccess/micropowermanager/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [x] I have searched the [issue tracker](https://github.com/EnAccess/micropowermanager/issues) for a feature request that matches the one I want to file, without success.

### Problem Description

Currently, transaction initiation is all over the place.

- SHS Sold Appliance modal has a Get Payment button
- Public Payment Webpages
- Some plugin implement custom Get Payment pages

Additionally, the verification logic is duplicated between base class and their implementation.

### Proposed Solution

Move to a single flow

### Alternatives Considered

N/A

### Additional Information

_No response_