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/default.nix

@@ -28,6 +28,7 @@
     ./mime.nix      # default applications (mimeapps.list), degrades with the suite
     ./recording.nix # nomarchy-record: screen recording behind Capture + the bar ⏺
     ./battery-notify.nix # low-battery toasts at the bar's 25/10% thresholds
+    ./first-boot.nix     # one-shot "you're set" toast on first session (#81)
     ./satty.nix     # satty screenshot annotation tool, themed
   ];