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

flake.nix

@@ -270,7 +270,6 @@
           "custom-mcrcon-rbw" = pkgs.custom.mcrcon-rbw;
           "custom-tea-rbw" = pkgs.custom.tea-rbw;
           "custom-rclone-torbox-setup" = pkgs.custom.rclone-torbox-setup;
-          "custom-nextcloud-talk-desktop" = pkgs.custom.nextcloud-talk-desktop;
           "custom-opencode" = pkgs.custom.opencode;
           "qt-pinned-jellyfin-media-player" = pkgsQt.jellyfin-media-player;
           "qt-pinned-stremio" = pkgsQt.stremio;
@@ -278,6 +277,7 @@
         // (
           if system == "x86_64-linux" then
             {
+              "custom-nextcloud-talk-desktop" = pkgs.custom.nextcloud-talk-desktop;
               # nix-deck kernel from Jovian-NixOS (Steam Deck) - expensive to build
               "nix-deck-kernel" = self.nixosConfigurations.nix-deck.config.boot.kernelPackages.kernel;
             }
@@ -341,30 +341,37 @@
           update-doomemacs = {
             type = "app";
             program = "${update-doomemacs}/bin/update-doomemacs";
+            meta.description = "Update Doom Emacs configuration";
           };
           update-claude-code = {
             type = "app";
             program = "${update-claude-code}/bin/update-claude-code";
+            meta.description = "Update Claude Code package version";
           };
           update-opencode = {
             type = "app";
             program = "${update-opencode}/bin/update-opencode";
+            meta.description = "Update OpenCode package version";
           };
           rotate-wallpaper = {
             type = "app";
             program = "${rotate-wallpaper}/bin/rotate-wallpaper";
+            meta.description = "Rotate desktop wallpaper";
           };
           upgrade = {
             type = "app";
             program = "${upgrade}/bin/upgrade";
+            meta.description = "Upgrade NixOS configuration";
           };
           bootstrap = {
             type = "app";
             program = "${bootstrap}/bin/bootstrap";
+            meta.description = "Bootstrap a new NixOS machine";
           };
           build-liveusb = {
             type = "app";
             program = "${build-liveusb}/bin/build-liveusb";
+            meta.description = "Build a bootable Live USB ISO";
           };
         }
       );