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.

home/desktop.nix

@@ -0,0 +1,123 @@
+# Graphical desktop layer: GUI apps, Wayland session env, and cursor theme.
+# Imported only on hosts that run Sway (MBP, T400, Mac Pro); never pulled onto
+# the headless WSL host. Login (and the Sway session launch) is handled by the
+# greetd/ReGreet greeter -- see ../modules/sway.nix -- so there is no tty1
+# autostart.
+{
+  pkgs,
+  config,
+  inputs,
+  identity,
+  ...
+}:
+{
+  imports = [
+    ./sway.nix
+  ];
+
+  home.packages = [
+    pkgs.element-desktop
+    pkgs.legcord
+    pkgs.nemo # file manager (launched via Mod+e, see ./sway.nix)
+    #pkgs.plex-desktop
+    #pkgs.plexamp
+  ];
+
+  home.sessionVariables = {
+    MOZ_USE_XINPUT2 = "1";
+    XDG_CURRENT_DESKTOP = "sway";
+  };
+
+  # Default apps for the desktop (writes ~/.config/mimeapps.list). Firefox owns
+  # the web; nemo owns directories/file URIs; images, PDFs and plain text open
+  # in Firefox too -- no dedicated GUI viewer/editor is installed and vim is
+  # terminal-only (no usable GUI .desktop for double-click handoff). Kept
+  # minimal -- only the handlers actually present on these hosts.
+  xdg.mimeApps = {
+    enable = true;
+    defaultApplications = {
+      "text/html" = "firefox.desktop";
+      "x-scheme-handler/http" = "firefox.desktop";
+      "x-scheme-handler/https" = "firefox.desktop";
+      "x-scheme-handler/about" = "firefox.desktop";
+      "x-scheme-handler/unknown" = "firefox.desktop";
+      "inode/directory" = "nemo.desktop";
+      "image/png" = "firefox.desktop";
+      "image/jpeg" = "firefox.desktop";
+      "image/gif" = "firefox.desktop";
+      "image/webp" = "firefox.desktop";
+      "image/svg+xml" = "firefox.desktop";
+      "application/pdf" = "firefox.desktop";
+      "text/plain" = "firefox.desktop";
+    };
+  };
+
+  # Theme GTK apps (nemo, etc.) to match the Catppuccin Mocha desktop. Under
+  # Sway there is no XSettings daemon, so GTK reads these from the generated
+  # ~/.config/gtk-{3,4}.0/settings.ini directly. The Mocha theme is dark by
+  # design, so no separate prefer-dark hint is needed.
+  gtk = {
+    enable = true;
+    # Theme GTK4 apps too (for any added later). GTK4 ignores gtk-theme-name,
+    # but home-manager turns this into an `@import` of the theme's
+    # gtk-4.0/gtk.css into ~/.config/gtk-4.0/gtk.css -- which even libadwaita
+    # honours, since that file overrides the named colours it uses
+    # (window_bg_color, accent_bg_color, ...). Set explicitly (same value as the
+    # legacy default) so it also silences the stateVersion<26.05 warning.
+    gtk4.theme = config.gtk.theme;
+    theme = {
+      name = "catppuccin-mocha-blue-standard";
+      package = pkgs.catppuccin-gtk.override {
+        accents = [ "blue" ];
+        variant = "mocha";
+      };
+    };
+    iconTheme = {
+      name = "Adwaita";
+      package = pkgs.adwaita-icon-theme;
+    };
+  };
+
+  home.pointerCursor = {
+    gtk.enable = true;
+    x11 = {
+      enable = true;
+      defaultCursor = "Adwaita";
+    };
+    package = pkgs.adwaita-icon-theme;
+    name = "Adwaita";
+    size = 24;
+  };
+
+  # Firefox is themed at the browser level (it does not follow the GTK theme).
+  # The system installs the binary (programs.firefox in ../modules/users.nix); here
+  # home-manager owns only the profile, hence package = null. Apply the
+  # Catppuccin Mocha theme add-on (only the mauve accent is packaged upstream;
+  # the rest of the desktop uses blue) and make content + UI dark.
+  programs.firefox = {
+    enable = true;
+    package = null;
+    # Keep the legacy profile location (~/.mozilla/firefox) -- that is where the
+    # system Firefox actually looks; pin it explicitly to silence the
+    # stateVersion<26.05 default-change warning (the new XDG path depends on
+    # Firefox's own profile support).
+    configPath = ".mozilla/firefox";
+    profiles.${identity.username} = {
+      id = 0;
+      isDefault = true;
+      extensions = {
+        force = true;
+        packages = [
+          inputs.firefox-addons.packages.${pkgs.stdenv.hostPlatform.system}.catppuccin-mocha-mauve
+        ];
+      };
+      settings = {
+        # Enable bundled add-ons automatically so the theme applies on first run.
+        "extensions.autoDisableScopes" = 0;
+        # Dark chrome + dark page content.
+        "ui.systemUsesDarkTheme" = 1;
+        "layout.css.prefers-color-scheme.content-override" = 0;
+      };
+    };
+  };
+}