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>

modules/home/options.nix

@@ -96,6 +96,33 @@ in
       description = "XKB variant for the Hyprland session.";
     };
 
+    keyboard.devices = lib.mkOption {
+      type = lib.types.attrsOf (lib.types.submodule {
+        options = {
+          layout = lib.mkOption {
+            type = lib.types.str;
+            description = "XKB layout for this specific keyboard.";
+          };
+          variant = lib.mkOption {
+            type = lib.types.str;
+            default = "";
+            description = "XKB variant for this keyboard.";
+          };
+        };
+      });
+      default = { };
+      example = lib.literalExpression ''{ "keychron-keychron-k2" = { layout = "de"; }; }'';
+      description = ''
+        Per-device keyboard layout. The key is the device name from
+        `hyprctl devices` (lower-case, hyphenated). Each entry generates a
+        Hyprland `device` block that overrides the session-wide
+        nomarchy.keyboard.layout for that keyboard only — e.g. an external
+        keyboard that's physically a different layout than the laptop's
+        built-in one. Hyprland applies it automatically whenever that
+        keyboard is connected.
+      '';
+    };
+
     package = lib.mkOption {
       type = lib.types.package;
       default = pkgs.nomarchy-theme-sync;