Refactor: Simplify module structure and reorganize services

- Removed system/ directory, merged applications into users/geir.nix
- Simplified fonts.nix to bare minimum (users can add more)
- Moved transmission.nix to sleeper-service/services/ (machine-specific)
- Organized grey-area services into services/ directory
- Updated import paths and tested all configurations
- Added research documentation for deploy-rs and GNU Stow

machines/sleeper-service/configuration.nix

@@ -7,7 +7,7 @@
     ./network-sleeper-service.nix
     # Services
     ./nfs.nix
-    ../../modules/system/transmission.nix
+    ./services/transmission.nix
 
     # User modules - server only needs sma user
     ../../modules/users/sma.nix