Merge reworked #13690: timezone-derived region and channel mirror prepends

Drop the .cn servers.sed twins, the USTC Arch fallback and the closed
#14399 timezone preselection. region.sh follows the installer's marker.

install/config/region.sh

@@ -4,7 +4,14 @@ cn_links=(
   "gemrc /etc/gemrc"
 )
 
-case ${OMARCHY_REGION:-global} in
+# The installer writes the marker; omarchy-region passes the region it switches to.
+region=global
+if [[ -e /etc/omarchy/region ]]; then
+  region=$(</etc/omarchy/region)
+fi
+region=${OMARCHY_REGION:-$region}
+
+case $region in
 global)
   rm -f /etc/omarchy/region
   sed -i 's/^LANG=zh_CN\.UTF-8$/LANG=en_US.UTF-8/' /etc/locale.conf
@@ -41,7 +48,7 @@ cn)
   fi
   ;;
 *)
-  echo "Unknown OMARCHY_REGION: $OMARCHY_REGION" >&2
+  echo "Unknown region: $region" >&2
   exit 1
   ;;
 esac