fix: human-readable Stellar error decoder with telemetry (closes #10)

[ZuLu0890]

Summary

Implements the full Stellar/Soroban error decoder required by issue #10, and fixes the TypeScript build error that was blocking compilation.

What was added

• src/data/errorCodes.json — comprehensive dictionary mapping every Stellar transaction/operation/Soroban error code to a human-readable message, troubleshooting steps, and documentation URL (ICU-style {placeholder} interpolation)
• src/utils/errorDecoder.ts — resolveError() function that extracts the error code from any raw Stellar SDK error shape, looks it up in the dictionary, interpolates context values, and falls through to a generic message for unmapped codes
• src/utils/errorTelemetry.ts — reportUnknownStellarError() that POSTs unknown codes to the telemetry endpoint (fire-and-forget, safe in SSR)
• src/hooks/useSorobanBilling.ts — React hook exposing decodeBillingError / billingError state
• src/hooks/useLedgerEvents.ts — React hook exposing decodeLedgerEventError / eventError state
• src/components/shared/ErrorToast.tsx — accessible client component with severity colour-coding and expandable troubleshooting section
• app/api/telemetry/stellar-errors/route.ts — Next.js route handler that receives unknown-code reports

Bug fixed

getErrorMessage() in errorDecoder.ts returns string | null | undefined, but UnknownErrorTelemetryPayload.message is typed as string | null. Added ?? null to coerce the value and resolve the TypeScript compile error.

Testing

npm run build now compiles and type-checks cleanly.

Closes #10

[ZuLu0890]

done