HttpErrorResponse implements the Error interface structurally but does not
extend the Error class, so `error instanceof Error` was always false for
HTTP failures and every travel error (400/409/500/network) fell through to
the same generic fallback string, hiding the backend's specific
TravelDomainError code/message (e.g. TRAVEL_ALREADY_ACTIVE) from the user.
toErrorMessage now checks `error instanceof HttpErrorResponse` first and
maps known travel error codes to specific German messages, falling back to
the generic message for unknown codes and to `error.message` for genuine
non-HTTP errors.
startTravel()'s catch block also now resyncs current travel state from the
server (reusing the existing pollCurrentTravel/refreshCurrentTravel path)
so a failed start caused by a race with another tab/request doesn't leave
the store's local state stale.
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