feat(renovate): update schedules to Saturday afternoon

- lockFileMaintenance: Saturday 2-4pm (was Monday 5am)
- nix-stable-ecosystem: Saturday 2-4pm
- nix-unstable-ecosystem: Saturday 2-4pm
- Add nixpkgs-qt rule: Saturday 4-6pm (staggered)

This allows CI builds to run overnight Saturday→Sunday, with human
review Saturday evening and builds complete by Sunday morning.

Closes: x-ymkgu

renovate.json

@@ -13,7 +13,7 @@
   "lockFileMaintenance": {
     "enabled": true,
     "schedule": [
-      "before 5am on monday"
+      "after 2pm and before 4pm on Saturday"
     ]
   },
   "dependencyDashboard": true,
@@ -37,6 +37,9 @@
         "/^nixpkgs$/",
         "/^home-manager$/",
         "/^nix-darwin$/"
+      ],
+      "schedule": [
+        "after 2pm and before 4pm on Saturday"
       ]
     },
     {
@@ -48,6 +51,21 @@
       "matchPackageNames": [
         "/nixpkgs-unstable/",
         "/home-manager-unstable/"
+      ],
+      "schedule": [
+        "after 2pm and before 4pm on Saturday"
+      ]
+    },
+    {
+      "description": "nixpkgs-qt updates on Saturday (staggered from main ecosystem)",
+      "matchManagers": [
+        "nix"
+      ],
+      "matchPackageNames": [
+        "/nixpkgs-qt/"
+      ],
+      "schedule": [
+        "after 4pm and before 6pm on Saturday"
       ]
     },
     {