fix(subscriptions): do not silently fall back to one-time payment on create failure (#15)

[ghzhost]

Closes #15

Problem

In useCreateSubscription, when subscriptionApi.buildCreateTransaction threw an error (for example 501 / backend unavailable / invalid params), the catch block silently fell back to building a plain, one-time buildPaymentTransaction locally and proceeded through signing and submission to subscriptionApi.create.

This meant the user unknowingly signed and submitted an immediate, full one-time payment instead of a recurring subscription schedule.

Solution

• Removed the silent fallback to buildPaymentTransaction in useCreateSubscription.
• Propagated any failure from subscriptionApi.buildCreateTransaction directly so the mutation transitions to error state and warns the user.
• Added comprehensive unit tests in src/hooks/useCreateSubscription.test.tsx verifying errors properly reject the mutation without calling buildPaymentTransaction or subscriptionApi.create.

Verification

• Ran test suite with NODE_ENV=test npm test: 28 test files passed (117 tests total).