Omit technical line wraps from displayed headers

Header lines may be wrapped for technical and standards compliance reasons. When we display message headers via the web, we currently include those technical line wraps intact. We should omit them. For example, we display:

https://www.openwall.com/lists/oss-security/2024/07/23/2

Subject: [OSSA-2024-002] OpenStack Nova: Incomplete file access fix and
 regression for QCOW2 backing files and VMDK flat descriptors
 (CVE-2024-40767)

but it should be:

Subject: [OSSA-2024-002] OpenStack Nova: Incomplete file access fix and regression for QCOW2 backing files and VMDK flat descriptors (CVE-2024-40767)

I thought this had something to do with:

                /* skip adjacent linear-white-space between previous
                 * encoded-word */
                r = --p;
                if (done != header) {
                        while (r > done && islinearwhitespace(*(--r)))
                                ;
                        if (r > done)
                                r = p;
                }
                buffer_append(dst, done, r - done);

and suspected this logic could be buggy or incomplete, but e.g. omitting if (r > done) r = p; made no difference.

Reproduce the example at the linked mailing-list archive URL and trace how folded headers move from parsing to web display. Inspect the header-unfolding logic around the shown linear-white-space code, then verify that the displayed Subject is a single line while header semantics remain intact.