waybar: drop unsupported CSS transform from tray drawer arrow

GTK 3 CSS (used by waybar) treats `transform` as a fatal parse error,
so the rotation approach killed waybar entirely. Use a color shift
from @tn-fg-muted to @tn-fg as the expanded-state feedback instead.
Matches the existing style.css convention (battery/cpu/temperature
states all use color shifts, not transforms).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

waybar/style.css

@@ -60,9 +60,9 @@ window#waybar {
 #custom-tray-arrow {
   padding: 0 12px;
   color: @tn-fg-muted;
-  transition: transform 200ms ease;
+  transition: color 200ms ease;
 }
 
 #group-tray-drawer.expanded #custom-tray-arrow {
-  transform: rotate(180deg);
+  color: @tn-fg;
 }