Fix nix flake check warnings and errors

- Move nextcloud-talk-desktop to x86_64-linux-only block (fixes aarch64 eval error)
- Remove duplicate deps from plasma-bigscreen (auto-injected by mkKdeDerivation)
- Replace nixfmt-rfc-style with nixfmt (deprecated alias)
- Set programs.git.signing.format = null (adopt new default)
- Add meta.description to all flake apps

home/roles/base/default.nix

@@ -60,6 +60,7 @@ in
 
     programs.git = {
       enable = true;
+      signing.format = null;
       settings = {
         user.name = "John Ogle";
         user.email = "john@ogle.fyi";