feat: pad code blocks, make the copy link a hover button

The copy link was a word-plus-glyph sitting above the block, always at
full strength, with no feedback once clicked. It is now an icon-only
button under the block, since Qt cannot float it over the tint.

Qt draws almost no box properties on <pre>: padding and vertical margins
are ignored, so the inset is margin-left/right and the room above and
below is a tinted spacer line. The glyph alone renders small whatever
font-size says, so the button is a tinted pill instead: grey at rest,
the highlight colour under the pointer, and a tick on green once used.
Qt rich text has no :hover, so hovering is QTextBrowser.highlighted()
driving a re-render, the same way the toggles already re-render.

The tick reverts after a moment rather than standing indefinitely, which
would claim the clipboard still holds that block long after it stopped
being true. Copying a second block first cancels the first one's revert.

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