Move Darwin-specific gc.interval to Darwin profile

Remove Darwin-specific gc.interval from base profile and add it to Darwin profile to fix NixOS compatibility.

🤖 Generated with [opencode](https://opencode.ai)

Co-Authored-By: opencode <noreply@opencode.ai>

profiles/base/default.nix

@@ -44,11 +44,6 @@
     };
     gc = {
       automatic = true;
-      interval = {
-        Weekday = 0;
-        Hour = 2;
-        Minute = 0;
-      };
       options = "--delete-older-than 30d";
     };
     extraOptions = ''

profiles/darwin/default.nix

@@ -14,6 +14,13 @@
     ./dock
   ];
 
+  # Darwin-specific gc interval format
+  nix.gc.interval = {
+    Weekday = 0;
+    Hour = 2;
+    Minute = 0;
+  };
+
   users.users.${user} = {
     name = "${user}";
     home = "/Users/${user}";