feat(session): first-boot welcome toast (#81)

One-shot user unit fires a dismissible "You're set" notification on the
first graphical session (SUPER+M / SUPER+T / SUPER+? + Network + Doctor),
then writes settings.firstBootShown into the flake checkout. Skips the
live ISO hostname (hosts/live.nix keeps its own toast). Toggle:
nomarchy.firstBootWelcome.enable. checks.first-boot covers the gate.

Verified: V2 — flake check --no-build; checks.first-boot green; local
shim smoke; downstream-template-home ships the unit.

modules/home/first-boot.nix

@@ -0,0 +1,37 @@
+# First-session welcome toast (nomarchy.firstBootWelcome) — one dismissible
+# "you're set" notification pointing at SUPER+M / SUPER+T / SUPER+? and
+# Network, then writes settings.firstBootShown into the flake checkout
+# (GOALS: no state outside the checkout). Live ISO keeps its own toast
+# (hosts/live.nix); the script self-skips on hostname nomarchy-live.
+{ config, lib, pkgs, ... }:
+
+{
+  config = lib.mkIf config.nomarchy.firstBootWelcome.enable {
+    systemd.user.services.nomarchy-first-boot = {
+      Unit = {
+        Description = "Nomarchy first-session welcome toast";
+        PartOf = [ "graphical-session.target" ];
+        # After the session (and preferably the notification daemon) is up.
+        After = [ "graphical-session.target" "graphical-session-pre.target" ];
+      };
+      Service = {
+        Type = "oneshot";
+        # PATH: real notify-send + theme-sync; VM check shims both via PATH.
+        # NOMARCHY_PATH matches home.sessionVariables so the marker lands
+        # in the user's flake checkout.
+        Environment = [
+          "PATH=${lib.makeBinPath [
+            pkgs.libnotify
+            pkgs.nomarchy-theme-sync
+            pkgs.coreutils
+          ]}"
+          "NOMARCHY_PATH=%h/.nomarchy"
+        ];
+        # Brief settle so swaync is accepting toasts (live ISO uses sleep 3).
+        ExecStartPre = "${pkgs.coreutils}/bin/sleep 2";
+        ExecStart = "${pkgs.nomarchy-first-boot}/bin/nomarchy-first-boot";
+      };
+      Install.WantedBy = [ "graphical-session.target" ];
+    };
+  };
+}