feat(menu): #79 slice 3 — Look & Feel › Auto theme (completes #79)

Adds an "Auto theme (on/off)" row to Look & Feel and an `autotheme`
submenu: toggle the feature, pick the day/night themes (from
`nomarchy-theme-sync list`), and set the sunrise/sunset times (HH:MM,
validated). This is the menu surface over settings.autoTheme, completing
the auto time-of-day theme feature (slices: CLI / timer / menu).

Enabling writes the flag and defaults the summer-day/summer-night pair
--no-switch, then runs `auto --force`, so a single rebuild both installs
the timer and applies the correct theme for now. Disabling is instant
(--no-switch): the flag flips and `auto` self-gates, so the lingering
timer no-ops until the next rebuild. Day/night/time writes are live
--no-switch (read by `auto` at tick time).

Verification: V1 — HM generation builds (bash -n on nomarchy-menu at
build) + bash -n on the built menu. Logic test with stubbed
rofi/theme-sync/notify: selecting the toggle from off runs exactly `set
day/night/enable ... --no-switch` then `auto --force` — the intended
one-rebuild path. V0 flake check green. All three #79 slices shipped;
the live timer switch across sunset is a real-session (V3) check on a
Nomarchy machine.

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

modules/home/rofi.nix

@@ -603,10 +603,15 @@ ${themeRows}
           then row "Night light (on)"  weather-clear-night
           else row "Night light (off)" weather-clear-night
           fi
+          if [ "$(nomarchy-theme-sync get settings.autoTheme.enable 2>/dev/null)" = true ]
+          then row "Auto theme (on)"  preferences-desktop-theme
+          else row "Auto theme (off)" preferences-desktop-theme
+          fi
           back
         } | rofi_menu -show-icons -markup-rows -p "Look & Feel") || exit 0
         case "$choice" in
           "$BACK")           exec "$0" ;;
+          *"Auto theme"*)    exec "$0" autotheme ;;
           *Theme*)           exec "$0" theme ;;
           *"Next wallpaper"*) exec nomarchy-theme-sync bg next ;;
           *"Reset wallpaper"*) exec nomarchy-theme-sync bg auto ;;
@@ -694,6 +699,67 @@ ${themeRows}
           && exec nomarchy-nightlight toggle
         notify-send "Night light" "Not available (nomarchy.nightlight off?)."; exit 0 ;;
 
+      autotheme)
+        # Look & Feel › Auto theme — automatic day/night theme switch
+        # (settings.autoTheme, applied by the nomarchy-auto-theme timer from
+        # autotheme.nix). The enable flag GATES the timer's install, so
+        # turning it *on* must rebuild; day/night/times are read live by
+        # `nomarchy-theme-sync auto`, so those writes are instant
+        # (--no-switch). Enabling writes the flag then `auto --force`, so a
+        # single rebuild both installs the timer and applies the right theme
+        # now. Disabling is instant: the flag flips and `auto` self-gates to
+        # a no-op, so the lingering timer does nothing until the next rebuild.
+        at_get() { nomarchy-theme-sync get "settings.autoTheme.$1" 2>/dev/null; }
+        en=$(at_get enable); day=$(at_get day); night=$(at_get night)
+        sunrise=$(at_get sunrise); sunset=$(at_get sunset)
+        choice=$( {
+          if [ "$en" = true ]
+          then row "Auto theme (on)"  preferences-desktop-theme
+          else row "Auto theme (off)" preferences-desktop-theme
+          fi
+          row "Day theme: ''${day:-— pick}"     weather-clear
+          row "Night theme: ''${night:-— pick}" weather-clear-night
+          row "Sunrise: ''${sunrise:-07:00}"    preferences-system-time
+          row "Sunset: ''${sunset:-20:00}"      preferences-system-time
+          back
+        } | rofi_menu -show-icons -markup-rows -p "Auto theme") || exit 0
+        case "$choice" in
+          "$BACK") exec "$0" lookfeel ;;
+          *"Auto theme"*)
+            if [ "$en" = true ]; then
+              nomarchy-theme-sync --quiet set settings.autoTheme.enable false --no-switch
+              notify-send "Auto theme" "Off — current theme stays."
+              exec "$0" autotheme
+            fi
+            # Need a day + night pair before enabling; default the summer pair.
+            [ -n "$day" ]   || nomarchy-theme-sync --quiet set settings.autoTheme.day summer-day --no-switch
+            [ -n "$night" ] || nomarchy-theme-sync --quiet set settings.autoTheme.night summer-night --no-switch
+            nomarchy-theme-sync --quiet set settings.autoTheme.enable true --no-switch
+            notify-send "Auto theme" "On — applying the theme for now…"
+            exec nomarchy-theme-sync auto --force ;;   # one rebuild: install timer + apply
+          "Day theme"*)
+            slug=$( { nomarchy-theme-sync list; printf '%s\n' "$BACK"; } | rofi_menu -p "Day theme") || exec "$0" autotheme
+            [ "$slug" != "$BACK" ] && [ -n "$slug" ] \
+              && nomarchy-theme-sync --quiet set settings.autoTheme.day "$slug" --no-switch
+            exec "$0" autotheme ;;
+          "Night theme"*)
+            slug=$( { nomarchy-theme-sync list; printf '%s\n' "$BACK"; } | rofi_menu -p "Night theme") || exec "$0" autotheme
+            [ "$slug" != "$BACK" ] && [ -n "$slug" ] \
+              && nomarchy-theme-sync --quiet set settings.autoTheme.night "$slug" --no-switch
+            exec "$0" autotheme ;;
+          "Sunrise"*|"Sunset"*)
+            key=sunrise; label=Sunrise; cur="''${sunrise:-07:00}"
+            case "$choice" in "Sunset"*) key=sunset; label=Sunset; cur="''${sunset:-20:00}" ;; esac
+            t=$(rofi_menu -p "$label HH:MM (now $cur)" < /dev/null) || exec "$0" autotheme
+            case "$t" in
+              [0-2][0-9]:[0-5][0-9])
+                nomarchy-theme-sync --quiet set "settings.autoTheme.$key" "$t" --no-switch ;;
+              "") : ;;
+              *) notify-send "Auto theme" "Ignored '$t' — use HH:MM (24-hour)." ;;
+            esac
+            exec "$0" autotheme ;;
+        esac ;;
+
       autotimezone)
         # Toggle automatic timezone detection (geoclue + automatic-timezoned).
         # A SYSTEM service, so flipping it writes the in-flake flag and runs a