feat(home-manager): Add platform compatibility guards to cross-platform roles

Add lib.optionals pkgs.stdenv.isLinux guards to roles that contain
Linux-only packages or services to prevent build failures on Darwin:

- communication: Guard Electron apps (element-desktop, fluffychat,
  nextcloud-talk-desktop) that don't build on Darwin due to electron
  build-from-source limitations
- kdeconnect: Guard entire config block since services.kdeconnect
  requires D-Bus and systemd (Linux-only)
- sync: Guard syncthingtray package (requires Linux system tray)
- email: Guard systemd.user.services/timers (Darwin uses launchd)
- desktop: Guard Linux-only packages, services, and KDE-specific
  configurations including gnome-keyring, systemd services, and
  XDG mime associations

Implements bead: nixos-configs-tcu

home/roles/communication/default.nix

@@ -4,6 +4,7 @@ with lib;
 
 let
   cfg = config.home.roles.communication;
+  isLinux = pkgs.stdenv.isLinux;
 in
 {
   options.home.roles.communication = {
@@ -12,14 +13,14 @@ in
 
   config = mkIf cfg.enable {
     home.packages = [
-      # Communication apps
+      # For logging back into google chat (cross-platform)
+      globalInputs.google-cookie-retrieval.packages.${system}.default
+    ] ++ optionals isLinux [
+      # Linux-only communication apps (Electron apps don't build on Darwin)
       pkgs.element-desktop
       # Re-enabled in 25.11 after security issues were resolved
       pkgs.fluffychat
       pkgs.nextcloud-talk-desktop
-      
-      # For logging back into google chat
-      globalInputs.google-cookie-retrieval.packages.${system}.default
     ];
   };
 }