Restructure networking configuration to per-machine modules

- Move networking configs to modules/network/ directory
- Create network-<machine-name>.nix files for each machine
- Add common.nix for shared networking configuration
- Update import paths in machine configurations
- Reduce duplication by using common networking settings

Network modules:
- modules/network/common.nix: Shared settings (nftables, SSH, tailscale)
- modules/network/network-congenital-optimist.nix: Workstation specific
- modules/network/network-sleeper-service.nix: File server specific

machines/congenital-optimist/configuration.nix

@@ -7,10 +7,10 @@
 }: {
   imports = [
     ./hardware-configuration.nix
+    ../../modules/network/network-congenital-optimist.nix
     
     # System modules
     ../../modules/system/fonts.nix
-    ../../modules/system/network.nix
     ../../modules/system/applications.nix
     
     # Hardware modules