New: resize the Thread panel

Drag the top-left corner of the panel to make it any size you like. The
panel remembers your size — across page loads, new tabs, and browser
restarts — so you only set it once.

It stays out of its own way: the panel can't cover the Thread buttons or
run off the top of the screen, and it shrinks to fit if you make the
browser window smaller. Widen the window again and your chosen size
comes back.