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>

hosts/live-iso.nix

@@ -1,6 +1,13 @@
 { config, pkgs, inputs, lib, ... }:
 
 {
+  # Live-ISO defaults. The install.sh keymap step calls `hyprctl keyword
+  # input:kb_layout` at runtime so the user's pick takes effect during the
+  # install, and writes the chosen value into the *installed* system.
+  console.keyMap = lib.mkDefault "us";
+  i18n.defaultLocale = lib.mkDefault "en_US.UTF-8";
+  services.xserver.xkb.layout = lib.mkDefault "us";
+
   # Home Manager activation for the nixos live user is provided by the
   # home-manager.nixosModules.home-manager module wired up in flake.nix.