Restructure modules/shared and update all imports

- Delete 2024-12-rework/ folder
- Rename modules/hosts/common to modules/shared, then split into:
  - modules/shared/home/general
  - modules/shared/home/linux
  - modules/shared/nixos
- Update all import paths throughout the codebase
- Update lib/*.nix to use new paths
- Fix hardcoded /Users/${username} to use config.home.homeDirectory
- Update README and examples/flake-structure.nix

modules/hosts/nixos/hetznix01/default.nix

@@ -1,6 +1,6 @@
 { inputs, pkgs, username,  ... }: {
   imports = [
-    ../../common/linux/nixroutes.nix
+    ../../../shared/nixos/nixroutes.nix
     ./disk-config.nix
     ./hardware-configuration.nix
     ./post-install

modules/hosts/nixos/hetznix01/post-install/default.nix

@@ -3,8 +3,8 @@
   restic_backup_time = "01:00";
 in {
   imports = [
-    ../../../common/linux/lets-encrypt.nix
-    ../../../common/linux/restic.nix
+    ../../../../shared/nixos/lets-encrypt.nix
+    ../../../../shared/nixos/restic.nix
     ./containers/emqx.nix
     ./matrix-synapse.nix
     ./monitoring.nix

modules/hosts/nixos/hetznix01/post-install/monitoring.nix

@@ -120,7 +120,7 @@ in {
       vmagent_push_pw = {
         owner = "vmagent";
         restartUnits = ["vmagent.service"];
-        sopsFile = ../../../common/secrets.yaml;
+        sopsFile = ../../../../shared/secrets.yaml;
       };
     };
   };