fix(home): pin xdg.userDirs.setSessionVariables — HM 26.05 default flip

home-manager 26.05 changes the default to false and warns on every
eval when home.stateVersion is older (every migrated machine). Pin the
legacy true: scripts and non-glib apps read the XDG_*_DIR session vars,
and an explicit value gives all downstreams identical behavior
regardless of stateVersion. mkDefault so a downstream can still opt out.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/default.nix

@@ -64,6 +64,11 @@
   xdg.userDirs = {
     enable = lib.mkDefault true;
     createDirectories = lib.mkDefault true;
+    # Pinned: HM 26.05 flips the default to false (and warns on every eval
+    # for older stateVersions). Keep exporting XDG_*_DIR into the session —
+    # scripts and non-glib apps read the vars, and pinning gives every
+    # downstream the same behavior regardless of its stateVersion.
+    setSessionVariables = lib.mkDefault true;
   };
 
   home.stateVersion = lib.mkDefault "26.05";