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>

modules/home/waybar.nix

@@ -16,6 +16,7 @@ let
   # is in play — multiple session layouts (comma-separated) or per-device
   # overrides (nomarchy.keyboard.devices) — so single-layout bars stay clean.
   showLanguage = lib.hasInfix "," config.nomarchy.keyboard.layout
+    || config.nomarchy.keyboard.layouts != [ ]
     || config.nomarchy.keyboard.devices != { };
 
   # Power-profile indicator (power-profiles-daemon). Both scripts self-