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/sync/default.nix

@@ -4,6 +4,7 @@ with lib;
 
 let
   cfg = config.home.roles.sync;
+  isLinux = pkgs.stdenv.isLinux;
 in
 {
   options.home.roles.sync = {
@@ -11,9 +12,10 @@ in
   };
 
   config = mkIf cfg.enable {
-    home.packages = with pkgs; [
+    # Linux-only: syncthingtray requires system tray support
+    home.packages = optionals isLinux (with pkgs; [
       syncthingtray
-    ];
+    ]);
 
     services.syncthing = {
       enable = true;