feat(keyboard): per-device layout for external keyboards

Add nomarchy.keyboard.devices ({ "<hyprctl-device-name>" = { layout; variant; }; })
generating Hyprland `device` blocks that override the session
nomarchy.keyboard.layout for a named keyboard -- e.g. an external board
that's physically a different layout than the laptop's built-in one.
Hyprland applies it whenever that device connects, so the external keyboard
"remembers" its layout the declarative way. A Waybar hyprland/language
indicator shows the active layout, placed only when more than one layout is
in play (a comma in the session layout, or any per-device override) so
single-layout bars stay clean.

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

README.md

@@ -188,6 +188,7 @@ examples: **[docs/OVERRIDES.md](docs/OVERRIDES.md)**.
 | `nomarchy.terminal` | `"ghostty"` | Terminal for keybinds and `$TERMINAL` |
 | `nomarchy.keyboard.layout` | `"us"` | XKB layout for the Hyprland session (installer writes the matching `services.xserver.xkb` in system.nix; the console + LUKS prompt follow via the distro default) |
 | `nomarchy.keyboard.variant` | `""` | XKB variant for the session |
+| `nomarchy.keyboard.devices` | `{}` | Per-device layout overrides (Hyprland `device` blocks keyed by `hyprctl devices` name) — e.g. an external keyboard with its own layout/variant |
 | `nomarchy.hyprland.enable` | `true` | Nomarchy's Hyprland config |
 | `nomarchy.waybar.enable` | `true` | Nomarchy's Waybar |
 | `nomarchy.rofi.enable` | `true` | Themed rofi launcher + `nomarchy-menu` dispatcher |