fix(gtk): add fill=#000000 to bundled submenu arrow SVG

GTK's -gtk-recolor requires the explicit string #000000 inside the SVG
to correctly substitute the foreground color. Without it, the shape defaults
to a hard un-recolorable black, which remained invisible against the dark
popover menu backgrounds. Added viewBox for robustness.

modules/home/stylix.nix

@@ -44,8 +44,8 @@ let
   # check marks: a bundled asset via -gtk-recolor(url(…)), immune to
   # icon-pack coverage. Shape traced from Adwaita's pan-end-symbolic.
   panEndSvg = pkgs.writeText "nomarchy-pan-end-symbolic.svg" ''
-    <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16">
-      <path d="M 6 4 L 10 8 L 6 12 Z"/>
+    <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+      <path fill="#000000" d="M 6 4 L 10 8 L 6 12 Z"/>
     </svg>
   '';
 in