feat(onboarding): stateful first-task onboarding flow (#418)

[Rufai-Ahmed]

Summary

Implements the stateful, fault-tolerant orchestration for the "create your first task" onboarding flow — the resilient core the issue emphasizes (#418).

What's included

• src/lib/onboarding/firstTaskFlow.ts — a pure reducer + helpers modelling the flow (create-task → fund-wallet → deploy-target). Each step tracks status (idle/running/success/error), attempts, and error; supports advance-on-success, error capture, retry, skip, reset, and flowProgress.
• src/lib/onboarding/useFirstTaskOnboarding.ts — a hook that runs each step's async action in order, advancing on success and surfacing failures for a manual retry() (plus skip()/reset()).

Design notes

• The side-effecting actions (fund testnet wallet, deploy mock target, create task) are passed in as a single injected runStep(stepId) function — so the orchestration is decoupled from the blockchain and fully unit-testable, and integrators wire the real Stellar calls.
• Complements (does not replace) the existing product-tour onboarding in src/lib/onboarding.

Scope

This delivers the stateful/fault-tolerant flow engine. The concrete testnet-funding (friendbot), mock-contract deploy, and onboarding UI are the integration follow-up — they plug into runStep and the exposed state.

Testing

14 Jest tests (reducer transitions + hook lifecycle: full run-through, failure→retry→resume, skip), all passing; tsc --noEmit clean for the new files.

Closes #418

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