feat: wrap code blocks and offer a copy link on each

Two things made a fenced code block awkward to read. Qt's markdown
stylesheet wraps p and li but not pre, and that stylesheet is dropped
when only the body fragment is kept, so a long line widened the whole
transcript and left it scrolling sideways. The pre style now carries
white-space:pre-wrap itself.

Copying a block meant selecting it by hand. Each block now gets a copy
link above it, on the right, in the same scheme-dispatch the thinking
and search toggles already use, since Qt rich text has no widgets to
put a real button in. The block text is read back from the source
markdown because Qt emits one <pre> per line, which makes the block
unrecoverable from the rendered HTML; _close_fences runs first, so a
block still streaming is copyable mid-reply. Like the other schemes,
a forged copy link in a reply is defused.

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