fix: button labels spill past their button #177

What

Button labels are not clipped to the button, so a label wider than its button spills over its neighbours. At 200% interface scale in a 1280x800 window, cellar's "Brian's Brain" and "Spaceships" buttons overflow into the next button (octalide/cellar#154).

Acceptance

• A button's label is clipped to the button, or ellipsized by the style's choice, using the truncation helpers from feat: text styles and rich spans #80
• The same holds for every widget that draws a label inside a box: segmented, tabs, dropdown, toggle