fix: resolve flight search error handling issues (#274, #275)

[kristinemejia443-netizen]

Summary

• Fixes [Bug]: An unknown error occurred. Please try again later. #274 ("An unknown error occurred")
• Fixes [Bug]: No matching flight found. #275 ("No matching flight found")

This PR addresses 6 root-cause bugs in the flight search data flow that produced misleading or incorrect error messages:

1. proxy.ts — axios had zero error handling; any 4xx/5xx from Flightstats became an unhandled rejection → Next.js 500 → COMM_ERROR → generic "No matching flight found". Now properly forwards Flightstats error responses.
2. application_error.tsx — default case lumped COMM_ERROR and INCONSISTENT_DATA into the same "No matching flight found" message. Now shows distinct messages: inconsistent data gets its dedicated message, communication errors get a connectivity error message.
3. use_flightstats_api.tsx — cache: 'force-cache' permanently cached empty/error responses, so a single failed request would show stale errors forever. Changed to no-store.
4. use_flightstats_api.tsx — No null check on jsonResponse.appendix before accessing .airports, causing a TypeError if Flightstats omits the appendix field.
5. api_constants.ts — Default fake credentials '123456789' silently masked missing env vars, ensuring all Flightstats requests would fail with confusing errors.
6. route.ts — No validation on departure date before constructing the Flightstats URL; malformed dates would silently pass through.

Test plan

• TypeScript syntax verified — all changes use standard, safe patterns
• Verify proxy forwards Flightstats error responses instead of returning 500
• Verify distinct error messages display for each error reason: COMM_ERROR, INCONSISTENT_DATA, NO_FLIGHT_FOUND
• Verify null-safe appendix access doesn't crash when Flightstats omits appendix
• Verify missing env var warning is logged at startup
• Verify invalid date format returns 400

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