refactor: increate font-weight of small nav items

Using a light font-weight on paragraph-size text decreases readability
and can be unexpectedly thin across different systems that have
different fonts installed. I've had some difficulty reading these for a
while and since this branch uses the default system font I think it's
probably a fine time to remove these light fonts.

Apple isn't the authority on good design, but their Human Interface
Guidelines document has a relevant note on font weight:

> **Use regular or heavy font weights in your app.** Use Regular, Medium, Semi-Bold, or Bold font weights, because they are easier to see. Avoid UltraLight, Thin, and Light font weights, which can be more difficult to see.
>
> -- https://developer.apple.com/design/human-interface-guidelines/accessibility/overview/text-size-and-weight/

Author: christianbundy

styles/base/main-window.mcss

@@ -76,7 +76,6 @@ MainWindow {
         padding: 4px 10px
         border-radius: 3px
         font-size: 120%
-        font-weight: 200
         cursor: pointer
         margin-left: 8px
         text-decoration: none !important
@@ -121,7 +120,6 @@ MainWindow {
       text-align: center
       font-size: 20px
       letter-spacing: 1px
-      font-weight: 200
       -webkit-app-region: drag
       position: relative
       span.title {