Fix menubar icon invisible in light mode (#741)

Set isTemplate = true on the menu bar icon image so macOS automatically
renders it black in light mode and white in dark mode. Changed the glyph
fill from white to black per template image convention.

Closes https://github.com/manaflow-ai/cmux/issues/737

Sources/AppDelegate.swift

@@ -7750,6 +7750,7 @@ enum MenuBarIconRenderer {
             drawBadge(text: text, in: config.badgeRect, config: config)
         }
 
+        image.isTemplate = true
         return image
     }
 
@@ -7778,7 +7779,7 @@ enum MenuBarIconRenderer {
         path.line(to: map(384.0, 369.0))
         path.close()
 
-        NSColor.white.setFill()
+        NSColor.black.setFill()
         path.fill()
     }