feat: setup flaypak.

flake.lock

@@ -481,6 +481,22 @@
         "type": "github"
       }
     },
+    "nix-flatpak": {
+      "locked": {
+        "lastModified": 1739444422,
+        "narHash": "sha256-iAVVHi7X3kWORftY+LVbRiStRnQEob2TULWyjMS6dWg=",
+        "owner": "gmodena",
+        "repo": "nix-flatpak",
+        "rev": "5e54c3ca05a7c7d968ae1ddeabe01d2a9bc1e177",
+        "type": "github"
+      },
+      "original": {
+        "owner": "gmodena",
+        "ref": "latest",
+        "repo": "nix-flatpak",
+        "type": "github"
+      }
+    },
     "nix-formatter-pack": {
       "inputs": {
         "nixpkgs": [
@@ -740,6 +756,7 @@
         "home-manager": "home-manager",
         "iamb": "iamb",
         "nbfc-linux": "nbfc-linux",
+        "nix-flatpak": "nix-flatpak",
         "nix-index-database": "nix-index-database",
         "nix-on-droid": "nix-on-droid",
         "nixpkgs": "nixpkgs_2",