refactor: split services.nix into named generators, remove dead tray option

Phase 4: Break the monolithic systemd.services expression into named
generator functions (mkPrepServices, mkVmServices, mkProxyServices,
mkConsoleRelayServices, mkConsoleScreenServices, mkVirtiofsdServices,
mkSoundServices, mkDbusProxyServices, mkWaylandSeccontextServices,
mkGpuServices, mkUsbServices, mkBalloondService). The top-level
expression is now a clean concatenation of named generators.

Phase 5: Remove unused vm.tray option (superseded by vm.dbus.tray).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/options.nix

@@ -264,20 +264,6 @@ let
           '';
         };
 
-        tray = lib.mkOption {
-          type = lib.types.submodule {
-            options = {
-              enable = lib.mkOption {
-                type = lib.types.bool;
-                default = false;
-                description = "Enable host-side tray proxy for this VM. Proxies guest SNI tray items to the host system tray.";
-              };
-            };
-          };
-          default = { };
-          description = "Tray proxy configuration for this VM.";
-        };
-
         autoShutdown = lib.mkOption {
           type = lib.types.submodule {
             options = {