niri: set XDG_CURRENT_DESKTOP=niri:GNOME for tray support

Many tray libraries (Electron's, Qt's StatusNotifier) gate icon
creation on a recognised XDG_CURRENT_DESKTOP value. Bare "niri"
makes them silently skip tray creation. Adding ":GNOME" as a
fallback identifier keeps niri primary while letting apps create
their tray icons.

Vesktop and Telegram are confirmed to want tray icons but were
not creating them; this fix unblocks both.

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

niri/config.kdl

@@ -31,6 +31,10 @@ input {
     }
 }
 
+environment {
+    XDG_CURRENT_DESKTOP "niri:GNOME"
+}
+
 spawn-at-startup "waybar"
 spawn-at-startup "mako"
 spawn-at-startup "awww-daemon"