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/options.nix

@@ -96,6 +96,22 @@ in
       description = "XKB variant for the Hyprland session.";
     };
 
+    keyboard.layouts = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ ];
+      example = [ "de" "fr" ];
+      description = ''
+        Extra candidate layouts offered by the interactive new-keyboard
+        picker. When non-empty, a small watcher runs in the session: when a
+        keyboard connects that isn't covered by nomarchy.keyboard.devices and
+        hasn't been chosen before, it pops a rofi picker (these layouts plus
+        the primary nomarchy.keyboard.layout), applies the choice, and
+        remembers it per-device (~/.local/state) — re-applying automatically
+        on later reconnects. The runtime-remember complement to the
+        declarative keyboard.devices; a stateful runtime piece by design.
+      '';
+    };
+
     keyboard.devices = lib.mkOption {
       type = lib.types.attrsOf (lib.types.submodule {
         options = {