Fix timer circle shrinking too much

Another small issue I discovered while writing !131 (merged). The timer circle would overlap with the content when the window is resized to very small sizes. It is better appreciable with the other one merged first.

Discussion:

• Micro change in "defaults": the default size of the circle is now a few pixels larger for the default window size (at least for me).
• For a small enough title, the radius shrinks when the window is small enough and the time passes the 1h mark (because the contents shrink).
• Computing the actual diagonal from the items' layout positions results in a circular binding detection, as the circle radius could have an impact on the layout itself. That's why I did a quick approximation using the heights and the maximum width.

Alternative: Setting a hard lower bound size depending on font size or something.