feat: extract seatd service to reusable module with boot log suppression

- Create modules/services/seatd.nix for clean greetd/tuigreet login experience
- Add boot log suppression options to prevent systemd messages on login screen
- Configure kernel parameters and journald to minimize console noise
- Update both little-rascal and congenital-optimist to use new seatd module
- Ensure consistent login experience across all machines
- Maintain compatibility with existing lab tool (binary name: lab)

modules/services/seatd.nix

@@ -0,0 +1,85 @@
+# Seatd service module with greetd display manager and boot log suppression
+# This module provides a clean login experience without systemd boot messages
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+with lib; {
+  options.services.seatd-clean = {
+    enable = mkEnableOption "seatd with greetd and clean boot";
+
+    suppressBootMessages = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Whether to suppress systemd boot messages that interfere with login screen";
+    };
+
+    tuigreetCommand = mkOption {
+      type = types.str;
+      default = "${pkgs.zsh}/bin/zsh";
+      description = "Command to execute after login via tuigreet";
+    };
+  };
+
+  config = mkIf config.services.seatd-clean.enable {
+    # Enable greetd display manager with tuigreet
+    services.greetd = {
+      enable = true;
+      settings = {
+        default_session = {
+          command = "${pkgs.greetd.tuigreet}/bin/tuigreet --time --cmd ${config.services.seatd-clean.tuigreetCommand}";
+          user = "greeter";
+        };
+      };
+    };
+
+    # Suppress systemd boot messages when enabled
+    boot = mkIf config.services.seatd-clean.suppressBootMessages {
+      # Kernel parameters to reduce boot noise
+      kernelParams = [
+        "quiet" # Reduce kernel messages
+        "systemd.show_status=false" # Hide systemd startup status
+        "rd.systemd.show_status=false" # Hide initrd systemd status
+        "rd.udev.log_level=3" # Reduce udev messages
+        "udev.log_level=3" # Reduce udev messages
+      ];
+
+      # Console configuration
+      consoleLogLevel = 0; # Minimum console log level
+
+      # Plymouth for clean boot (optional)
+      plymouth = {
+        enable = false; # Keep disabled for now, can be enabled if desired
+      };
+    };
+
+    # Systemd and journald configuration to reduce noise
+    systemd = mkIf config.services.seatd-clean.suppressBootMessages {
+      # Reduce log levels
+      extraConfig = ''
+        LogLevel=warning
+        DefaultStandardOutput=null
+        DefaultStandardError=null
+      '';
+    };
+
+    # Journald configuration
+    services.journald = mkIf config.services.seatd-clean.suppressBootMessages {
+      extraConfig = ''
+        # Reduce console output
+        ForwardToConsole=no
+        MaxLevelConsole=warning
+        # Keep reasonable log retention
+        MaxRetentionSec=7day
+        SystemMaxUse=500M
+      '';
+    };
+
+    # Ensure proper TTY configuration for clean display
+    console = {
+      earlySetup = true;
+    };
+  };
+}