Use font-weight 500 for interactive controls

Interactive controls paired with icons should use weight 500 instead of bold (700) or regular (400) to roughly visually match our icons' weight. This is of course font-dependent, but 500 seems to work pretty well generally. Example on Safari: <img width="250" height="129" alt="Image" src="https://github.com/user-attachments/assets/ccd3c9d3-0d4b-4d96-9a4a-f99af7f1ea32" /> Currently, I think this only concerns: - `NcButton` - The main `label` of `NcFormBox*` components