feat(message): show From/To/Cc and add a details dialog

MimeParser has filled To and Cc all along and HtmlBuilder simply never
interpolated them, so both were parsed on every message and then
discarded. The header strip showed the subject and a message count and
nothing else, which is item 2 of the usability backlog.

The header now adapts to what it can say honestly. A thread holding one
message shows From, To and Cc under the subject, where every field is
unambiguous. A thread holding several keeps showing the subject and the
count alone: the recipient differs message to message, and once the user
has replied there is no single address the thread is addressed to, so
naming one would be a guess presented as a fact. Per-message detail is
what the dialog is for.

That dialog lists Subject, From, To, Cc, Date and Message-Id for every
message, numbered when there is more than one, in a read-only plain-text
widget. Plain text is the security decision, not a stylistic one: these
values come from strangers and the dialog exists to show them verbatim,
so the format that cannot interpret markup is the right one. The header
label is RichText and every value interpolated into it is escaped, since
an unescaped From injects into the application's own chrome rather than
into the sandboxed page.

Reached by a Details... button beside the subject and by Ctrl+Shift+D.
Both, because a shortcut alone restates the complaint this backlog opened
with. The binding is shifted because Ctrl+D is delete, and the
destructive action keeps the key it already had rather than being moved
to make room.

An empty Cc omits its row instead of printing a label with nothing after
it. Both header shapes were rendered to PNG and inspected, not only
asserted.

The new button also exposed a latent flaw in an older test:
attachmentButtonLabels() identified attachment buttons by excluding the
one other button's label, so it counted the details button as an
attachment as soon as one existed. It now finds the bar by object name
and reads only its children.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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