fix: propagate and handle NO_FLIGHT_FOUND API errors properly

[VQToan]

Fixes #275

Description

Preserves structured purchase-validation failures from the API route to the client. This surfaces specific messages like No matching flight found instead of a generic purchase failure, which occurs when the API route collapses validation failures into generic HTTP 500 errors or unhandled client-side exceptions.

Changes

• Added custom FlightNotFoundError and InconsistentFlightDataError to errors.ts.
• Updated purchase/route.ts to throw these typed errors during validateFlightPlan when flight stats are missing or mismatched, and to return structured JSON responses with specific error codes.
• Updated use_local_api.tsx to parse these JSON responses and throw Error instances annotated with err.reason.
• Updated use_application.tsx's catch block to look for err.reason === Reason.NO_FLIGHT_FOUND and Reason.INCONSISTENT_DATA, dispatching the exact translated UI error messages.
• Fixed a bug where setError couldn't take a reason field in flightData.ts.

This PR was created with AI assistance.