feat(flake): add nixpkgs-qt input for qt5webengine packages

Add separate nixpkgs input for qt5webengine-dependent packages like
jellyfin-media-player. This input updates on a separate Renovate
schedule from main nixpkgs to avoid massive qt5webengine rebuilds
when updating other packages.

- Add nixpkgs-qt input pinned to nixos-25.11
- Create pkgs.qt-pinned overlay namespace

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.lock

@@ -258,6 +258,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-qt": {
+      "locked": {
+        "lastModified": 1770464364,
+        "narHash": "sha256-z5NJPSBwsLf/OfD8WTmh79tlSU8XgIbwmk6qB1/TFzY=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "23d72dabcb3b12469f57b37170fcbc1789bd7457",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-25.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs-unstable": {
       "locked": {
         "lastModified": 1769170682,
@@ -347,6 +363,7 @@
         "nix-darwin": "nix-darwin",
         "nix-doom-emacs-unstraightened": "nix-doom-emacs-unstraightened",
         "nixpkgs": "nixpkgs",
+        "nixpkgs-qt": "nixpkgs-qt",
         "nixpkgs-unstable": "nixpkgs-unstable",
         "perles": "perles",
         "plasma-manager": "plasma-manager",