refactor(flake): add user registry and multi-user host support

Separate user identity (data) from the reusable modules, and let a host
declare any number of users instead of exactly one.

- users/registry.nix: per-user identity (name, email, groups, authorized
  and signing keys) as the single source of identity; no user data is
  hardcoded in the modules.
- mkHost takes a `users` set keyed by username; per-user identity is
  injected into each home config via the `identity` module arg
  (extraSpecialArgs is per-host, so it cannot carry per-user data).
- modules/users.nix builds accounts from the registry; modules/ssh.nix no
  longer defines authorized keys (the registry owns them); home/git.nix
  and home/desktop.nix read `identity`; users/emmathorpe/work.nix drops
  its now-redundant git identity override.
- Restructure the tree: users/, home/, modules/, hosts/, lib/ replace the
  former lyrathorpe/ and system/ layout.
- Add standalone homeConfigurations (the portable subset: shell, git,
  editor, claude) and an exported homeModules output for use on machines
  not managed by this flake, or as an input to other flakes.

Behaviour-preserving for existing hosts: lyrathorpe-mbp and
emmathorpe-edaas evaluate to identical derivations; lyrathorpe-t400,
lyrathorpe-macpro31 and lyrathorpe-rpi5 differ only by de-duplicating a
repeated authorized_keys entry. Fixes the SSH authorized-key leak (one
user's key was applied to every account), the hardcoded default git
identity, and the hardcoded EDaaS linger setting.

lib/catppuccin-mocha.nix

@@ -0,0 +1,26 @@
+# Catppuccin Mocha palette. Raw 6-digit hex (no leading "#"); consumers add a
+# "#" where their format needs it. Shared by the Sway desktop theming
+# (home/sway.nix) and the ReGreet greeter (modules/sway.nix) so the two stay in sync.
+{
+  base = "1e1e2e";
+  mantle = "181825";
+  crust = "11111b";
+  surface0 = "313244";
+  surface1 = "45475a";
+  surface2 = "585b70";
+  overlay0 = "6c7086";
+  subtext0 = "a6adc8";
+  subtext1 = "bac2de";
+  text = "cdd6f4";
+  rosewater = "f5e0dc";
+  red = "f38ba8";
+  maroon = "eba0ac";
+  peach = "fab387";
+  yellow = "f9e2af";
+  green = "a6e3a1";
+  teal = "94e2d5";
+  sapphire = "74c7ec";
+  blue = "89b4fa";
+  mauve = "cba6f7";
+  pink = "f5c2e7";
+}