Align the China region to the official cn profile

The region value is cn, the China files live under default/regions/cn/, and
omarchy-apply-pacman renders pacman for every caller: the .cn twins of the
channel servers first, then .org, USTC and ArchLinuxCN. Switching to China
installs and trusts the ArchLinuxCN keyring. A migration renames china to cn
and repoints the drop-in links.

install/config/region.sh

@@ -1,4 +1,4 @@
-china_links=(
+cn_links=(
   "environment.conf /etc/environment.d/90-omarchy-china.conf"
   "cargo.toml /.cargo/config.toml"
   "gemrc /etc/gemrc"
@@ -8,17 +8,17 @@ case ${OMARCHY_REGION:-global} in
 global)
   rm -f /etc/omarchy/region
   sed -i 's/^LANG=zh_CN\.UTF-8$/LANG=en_US.UTF-8/' /etc/locale.conf
-  for link in "${china_links[@]}"; do
+  for link in "${cn_links[@]}"; do
     read -r _ target <<<"$link"
-    if [[ -L $target && $(readlink "$target") == */default/region/china/* ]]; then
+    if [[ -L $target && $(readlink "$target") == */default/regions/cn/* ]]; then
       rm "$target"
     fi
   done
   ;;
-china)
+cn)
   # Packages first, so a failed install leaves the machine Global.
   omarchy-pkg-add fcitx5-configtool fcitx5-rime librime opencc rime-ice-installer
-  for link in "${china_links[@]}"; do
+  for link in "${cn_links[@]}"; do
     read -r _ target <<<"$link"
     if [[ -e $target && ! -L $target ]]; then
       echo "Refusing to replace $target, which Omarchy did not create" >&2
@@ -26,11 +26,11 @@ china)
     fi
   done
   mkdir -p /etc/omarchy
-  echo china >/etc/omarchy/region
-  for link in "${china_links[@]}"; do
+  echo cn >/etc/omarchy/region
+  for link in "${cn_links[@]}"; do
     read -r source target <<<"$link"
     mkdir -p "${target%/*}"
-    ln -sfn "$OMARCHY_PATH/default/region/china/$source" "$target"
+    ln -sfn "$OMARCHY_PATH/default/regions/cn/$source" "$target"
   done
   sed -i 's/^#\(zh_CN\.UTF-8 UTF-8\)/\1/' /etc/locale.gen
   locale-gen

install/provisioning/setup-form.sh

@@ -129,12 +129,15 @@ omarchy_prompt_region() {
     china="China 中国"
     header="Select region · 选择区域"
   fi
-  [[ $region == "china" ]] && selected=$china
+  [[ $region == "cn" ]] && selected=$china
   choice=$(printf '%s\n' Global "$china" | gum choose --selected "$selected" --header "$header") && status=0 || status=$?
   ((status == 0)) || return $status
 
-  region=${choice%% *}
-  region=${region,,}
+  if [[ $choice == "Global" ]]; then
+    region=global
+  else
+    region=cn
+  fi
 }
 
 omarchy_prompt_keyboard() {