feat: Vikunja to-do app (#128)

Vikunja is fast self-hostable to-do app.

Reviewed-on: https://git.selfprivacy.org/SelfPrivacy/selfprivacy-nixos-config/pulls/128
Reviewed-by: Inex Code <inex.code@selfprivacy.org>
Co-authored-by: nhnn <nhnn@nhnn.dev>
Co-committed-by: nhnn <nhnn@nhnn.dev>

sp-modules/vikunja/flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs-24-11": {
+      "locked": {
+        "lastModified": 1744440957,
+        "narHash": "sha256-FHlSkNqFmPxPJvy+6fNLaNeWnF1lZSgqVCl/eWaJRc4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "26d499fc9f1d567283d5d56fcf367edd815dba1d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-24.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs-24-11": "nixpkgs-24-11"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}