qtui: Properly render hovered-upon clickables

qtui: Properly render hovered-upon clickables

Additional formats given to QTextLayout::draw() are not cumulative.
Since we were using this feature for rendering both selections and
active (hovered-upon) clickables, things would break when both
overlapped. In particular, a highlighted *and* selected URL would
vanish while being hovered.

Fix this by explicitly calculating the range of affected formats
for both selections and active clickables, taking overlap into
account. Remove special handling in ContentsChatItem in favor of
the more generic solution.

Use the newly-introduced "hovered" message label to determine how
an active clickable should be rendered, rather than hardcoding
an underline.