fix(desktop): point ~/.icons/default at the cursor theme

Steam gets XCURSOR_THEME from the session variables and still drew the
core pointer: parts of it resolve the theme literally named 'default'
instead of reading the variable. Xcursor looks that name up along
XCURSOR_PATH, where ~/.icons comes first, so a link there covers those
lookups as well.

modules/desktop.nix

@@ -408,9 +408,8 @@ in
   environment.sessionVariables.QT_STYLE_OVERRIDE = "kvantum";
 
   # Same gap, cursors: niri's cursor block reaches xwayland-satellite, so X11
-  # clients have a themed pointer available, but Steam is started by the user
-  # manager and arrives with XCURSOR_PATH alone. Xcursor then falls back to
-  # ~/.icons/default, which does not exist here, and draws the core theme.
+  # clients have a themed pointer available, but everything under the user
+  # manager arrives with XCURSOR_PATH alone and has no theme name to look up.
   # The name matches modules/greetd.nix, so greeter and session agree.
   environment.sessionVariables.XCURSOR_THEME = "Sweet-cursors";
   environment.sessionVariables.XCURSOR_SIZE = "24";
@@ -429,5 +428,13 @@ in
     # otherwise ignore the monitor.kdl it writes next to it.
     "C %h/.config/niri/config.kdl 0644 - - - ${niriUserConfig}"
     "C %h/.zshrc 0644 - - - ${userZshrc}"
+    # Steam gets XCURSOR_THEME from the block above — verified in its own
+    # environment — and still drew the core pointer, so parts of it resolve the
+    # theme literally named 'default' rather than the variable. Xcursor looks
+    # that name up along XCURSOR_PATH like any other, and ~/.icons is first,
+    # so pointing it at the theme covers those lookups too. 'L+' replaces the
+    # link on every activation, unlike the 'C' rules above, which keeps it
+    # following the theme instead of freezing at today's one.
+    "L+ %h/.icons/default - - - - /run/current-system/sw/share/icons/Sweet-cursors"
   ];
 }