Refactor flake targets into platform x build-type matrix

Generates all nixosConfigurations from mkTarget(platform, buildType,
hostPath) instead of hand-written per-host blocks, so adding a new
platform or build type is a one-line addition. Per-machine identity
(hostname, hostId, secrets, stateVersion) moves into hosts/<name>/host.nix;
platform-specific config (hardware, boot, networking) into
modules/platforms/*.nix; build-type config (minimal/server/docker/gui/
nix-cache/pxe-boot) into modules/build-types/*.nix.

Old flat targets (nixos, docker, server, nix-cache, nix-minimal, pxe-boot)
are replaced by the 17-target <platform>-<buildtype> matrix; each new
target was verified to evaluate before its old counterpart was removed.
CI workflows and docs/aliases now discover hosts dynamically via
nixosConfigurations attrNames and /etc/flake-target instead of hardcoded
lists, so they can't drift from flake.nix again.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

modules/build-types/gui.nix

@@ -0,0 +1,57 @@
+{ config, pkgs, lib, inputs, ... }:
+
+{
+  environment.systemPackages = with pkgs; [
+    inputs.nixos-conf-editor.packages.${pkgs.system}.nixos-conf-editor
+    nodejs
+    appimage-run
+    seahorse
+    vscode
+    p7zip
+    popsicle # balena-etcher
+    shotcut
+    gimp
+    pdfarranger
+    terminator
+    libreoffice-qt
+    transmission_4-qt
+  ];
+
+  boot.loader.grub.useOSProber = true;
+
+  services.xserver.enable = true;
+  services.xserver.displayManager.lightdm.enable = true;
+  services.xserver.desktopManager.cinnamon.enable = true;
+
+  services.xserver.xkb = {
+    layout = "au";
+    variant = "";
+  };
+
+  services.printing.enable = true;
+
+  security.rtkit.enable = true;
+  services.pipewire = {
+    enable = true;
+    alsa.enable = true;
+    alsa.support32Bit = true;
+    pulse.enable = true;
+  };
+
+  users.users.nixos.extraGroups = [ "networkmanager" ];
+
+  programs.firefox.enable = true;
+
+  services.xrdp.enable = true;
+  services.xrdp.defaultWindowManager = "cinnamon-session";
+  services.xrdp.openFirewall = true;
+  nixpkgs.config.allowUnfree = true;
+
+  services.gnome.gnome-keyring.enable = true;
+  security.pam.services.login.enableGnomeKeyring = true;
+
+  services.xserver.displayManager.sessionCommands = ''
+    eval $(gnome-keyring-daemon --start --components=secrets,ssh)
+    export SSH_AUTH_SOCK
+  '';
+}