feat(keyboard): interactive new-keyboard layout prompt + persist

The runtime-remember complement to nomarchy.keyboard.devices. Set
nomarchy.keyboard.layouts (candidate layouts) and a nomarchy-keyboard-watch
daemon runs (exec-once): it polls hyprctl devices, and when a keyboard
connects after login that isn't declared and hasn't been chosen before, it
pops a rofi layout picker, applies the choice via hyprctl switchxkblayout (an
index into the candidate set carried by input.kb_layout), and remembers it
per-device in ~/.local/state -- re-applied silently on later reconnects. The
boot-time set (incl. the built-in keyboard) is never prompted.

Pure bash + jq/hyprctl/rofi (all in the session PATH); state lives outside
the flake (a stateful runtime piece by design). Eval + bash-syntax verified;
the hotplug behaviour needs an on-hardware test.

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

README.md

@@ -189,6 +189,7 @@ examples: **[docs/OVERRIDES.md](docs/OVERRIDES.md)**.
 | `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.keyboard.layouts` | `[]` | Extra candidate layouts; when set, a watcher prompts (rofi) for a layout on a newly-connected keyboard and remembers it per-device |
 | `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 |