feat: keymap/locale + form factor in installer; nm-applet visible by default

- Installer prompts for keyboard layout (with optional variant) and locale
  via curated short list + Other… fallback into the full localectl list;
  applies to the live session immediately (loadkeys + hyprctl) so the
  rest of the install types correctly. Generated system.nix emits
  console.keyMap, i18n.defaultLocale, and services.xserver.xkb.{layout,
  variant}.
- New nomarchy.{system,}.formFactor enum (laptop|desktop, default laptop).
  Installer auto-detects via /sys/class/power_supply/BAT* and lets the
  user flip the answer. Waybar drops the battery widget on desktop;
  battery-monitor service is gated on the same option.
- Lift waybar tray out of the collapsed group/tray-expander in the default
  theme so nm-applet's icon is visible without expanding the drawer.
- Live ISOs (TTY + graphical) get baseline mkDefault keyMap/locale so the
  installer's runtime override always wins.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/home/options.nix

@@ -44,6 +44,18 @@
       default = "summer-night";
       description = "System theme name.";
     };
+    formFactor = lib.mkOption {
+      type = lib.types.enum [ "laptop" "desktop" ];
+      default = "laptop";
+      description = ''
+        Physical form factor. Drives UI affordances (battery widget,
+        future lid handling). Default "laptop" — battery widget is
+        harmless on a desktop (renders empty when no BAT* is present),
+        so the safe default is "show, don't hide". The installer
+        auto-detects via /sys/class/power_supply/BAT* and writes the
+        explicit value into the generated home.nix.
+      '';
+    };
     wallpaper = lib.mkOption {
       type = lib.types.str;
       default = "";