feat(i18n): proper multilingual input

Use fcitx5 to switch between different inputs. Installed mozc-ut for
Japanese input with the bepo layout, as well as fcitx5-chinese to load
table-defined layouts for the IPA/X-SAMPA input method.

Currently, mozc_server and fcitx5 need to be launched by the
compositor (only Hyprland for now), auto-launching from module options
needs to be implemented in the future.

system/i18n/input.nix

@@ -0,0 +1,24 @@
+{
+  lib,
+  config,
+  ...
+}:
+with lib; let
+  cfg = config.mySystem.i18n.input;
+in {
+  options.mySystem.i18n.input.enable = mkEnableOption "Enable i18n input with fcitx5";
+
+  config.i18n.inputMethod = mkIf cfg.enable {
+    enable = true;
+    type = "fcitx5";
+    fcitx5.addons = with pkgs; [
+      fcitx5-gtk
+      fcitx5-mozc-ut # Japanese input support
+      fcitx5-nord
+      fcitx5-table-other # X-SAMPA to IPA support
+      qt6Packages.fcitx5-chinese-addons # allow to load table addons
+      qt6Packages.fcitx5-configtool
+      qt6Packages.fcitx5-with-addons
+    ];
+  };
+}