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/autotheme.nix

@@ -0,0 +1,36 @@
+# Auto time-of-day theme switch (BACKLOG #79, VISION § D). A user timer
+# periodically runs `nomarchy-theme-sync auto`, which reads
+# settings.autoTheme = { enable, day, night, sunrise, sunset } from the
+# state file and applies the day or night preset for the current clock —
+# through the SAME one engine as a manual `apply` (no second pipeline).
+#
+# The command self-gates (no-op when disabled) and is idempotent (it only
+# rebuilds when the active theme actually needs to change), so the timer
+# can tick freely. Install is gated on the state flag (like nomarchy.updates
+# gates on its enable), so a machine not using the feature carries no timer;
+# enabling it from the menu (slice 3) writes the flag + rebuilds, which is
+# what brings the timer into being.
+{ config, lib, pkgs, ... }:
+
+lib.mkIf (config.nomarchy.settings.autoTheme.enable or false) {
+  systemd.user.services.nomarchy-auto-theme = {
+    Unit.Description = "Apply the day/night theme for the current time";
+    Service = {
+      Type = "oneshot";
+      # The rebuild (`home-manager switch`) and its tools resolve from the
+      # system + per-user profiles — same PATH shape nomarchy-updates uses.
+      Environment = "PATH=/run/current-system/sw/bin:/etc/profiles/per-user/${config.home.username}/bin";
+      ExecStart = "${pkgs.nomarchy-theme-sync}/bin/nomarchy-theme-sync auto";
+    };
+  };
+
+  systemd.user.timers.nomarchy-auto-theme = {
+    Unit.Description = "Periodic day/night theme check";
+    Timer = {
+      OnStartupSec = "1min";   # settle on the right theme shortly after login
+      OnCalendar = "*:0/15";   # re-check every 15 min; auto no-ops if unchanged
+      Persistent = true;       # catch a transition missed while powered off
+    };
+    Install.WantedBy = [ "timers.target" ];
+  };
+}

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