feat(autotheme): #79 slice 2 — systemd timer runs auto

modules/home/autotheme.nix installs a user service + timer
(nomarchy-auto-theme) that periodically runs `nomarchy-theme-sync auto`,
so the day/night theme is applied on the current schedule through the
same one engine. Install gates on settings.autoTheme.enable (read from
state, the way updates.nix gates on its enable), so a machine not using
the feature carries no timer.

Timer: OnStartupSec=1min (settle after login) + OnCalendar=*:0/15 +
Persistent (catch a transition missed while off). Periodic rather than
exact sunrise/sunset timers — robust to suspend/DST, and `auto` is
idempotent (no rebuild unless the theme must change) so ticks are cheap.

Verification: V1 — real-config eval (state temporarily enabled, then
restored) confirms the units land when enabled (OnCalendar *:0/15,
ExecStart `... nomarchy-theme-sync auto`) and are absent when disabled;
systemd-analyze validates the schedule. V0 flake check green. The live
timer -> home-manager switch on a real Nomarchy session is pending a
Nomarchy machine (same blocker as #76).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home/default.nix

@@ -18,6 +18,7 @@
     ./yazi.nix      # flagship TUI file manager, themed + plugins
     ./osd.nix       # swayosd volume/brightness OSD, themed
     ./nightlight.nix # scheduled blue-light filter (hyprsunset), opt-in
+    ./autotheme.nix  # auto day/night theme switch (settings.autoTheme), opt-in
     ./timezone.nix  # keep the Waybar clock in step with auto-timezone changes
     ./updates.nix   # passive update-awareness indicator + notification, opt-in
     ./shell.nix     # zsh + starship + bat/eza/zoxide, themed