Add account funding pre-flight checker

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Before letting a user attempt to pay/create an invoice, check whether their account exists and has sufficient balance for the operation plus fees, surfacing a clear "fund your account first" UX hook rather than a late on-chain failure.

Technical Context

New src/preflightChecker.ts. Uses server.getAccount plus balance checks against the required token, called proactively by UI integrators before invoking client.pay/client.createInvoice.

Acceptance Criteria

• checkPayerReadiness(address, requiredAmount, token) returns { ready: boolean, reason?: "account_not_found"|"insufficient_balance"|"no_trustline" }
• Distinguishes the three failure reasons accurately
• Exported from src/index.ts
• Tests: each reason case individually verified; ready case returns ready: true with no reason

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[collinsezedike]

Let me help you with this issue, Kingsman.

I have experience building preflight validation checks for on-chain operations and know the ordering matters here: account existence, then trustline, then balance, since each failure mode needs its own distinct check.

Here's my plan:

• Implement checkPayerReadiness(address, requiredAmount, token) in src/preflightChecker.ts
• Check account existence via server.getAccount first, returning account_not_found on failure
• Check trustline presence for the required token, returning no_trustline if absent
• Check balance against requiredAmount plus fees, returning insufficient_balance if short
• Return { ready: true } with no reason when all checks pass
• Export from src/index.ts
• Tests verifying each failure reason independently and the ready case

All acceptance criteria covered. Happy to start immediately I am assigned.