fix(site): improve error handling for /contact

[BaoT1301]

Description

Fixes #740

The public inquiry action on /contact forwarded the caught error's raw message directly to the client:

throwActionError(err instanceof Error ? err.message : 'An unknown error occurred.');

That can leak internal details to end users (upstream HTTP status codes, a missing CONTACT_GOOGLE_SCRIPT_URL config error, or other exception text that should never leave the server).

What changed

• The full error is still logged server-side with logger.error for diagnosis.
• The caller now always receives a fixed, generic message ("We could not submit your inquiry right now. Please try again later."), regardless of the underlying failure.

Tests

• Updated the failure-path test to assert the generic message is thrown, that the raw internal message (Sheets is down) is not surfaced to the caller, and that the full internal error is still logged server-side.

No UI or behavior change beyond the error text the user sees on a failed submission.

Checklist

• You've included unit or integration tests for your change, where applicable.
• You've included inline docs for your change, where applicable.
• Any components that you've modified are accessible.
• You've used conventional commits where appropriate

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