Add loading, error, and toast states

[iexwr]

Context

Forms and fetches have no user feedback; failures are silent and success is unclear.

Acceptance criteria

• Add a loading indicator on async actions (create, pay, fetch).
• Surface API/contract errors in a visible banner/message.
• Show a success toast on create/pay completion.
• No unhandled promise rejections.

Technical notes

• Keep it dependency-light (a tiny toast component, no heavy UI lib).

Testing

• Test that error and success paths render the correct UI.