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/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() {