Region setting (WiFi country, locale, keyboard) on the Super+W screen and Web UI

kiosk-wifi-helper gains region-status/region-set. The country is persisted as
a cfg80211 regdom module option and in the WiFi netplan file; locale and
keyboard go through localectl, validated against what's installed.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A97KUggSE3rBL8veDM5Mr4

webui/test/fixtures/fake-wifi-helper.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+# Stand-in for kiosk-wifi-helper in api.test.js: echoes the action and stdin.
+input=$(cat)
+case "$1" in
+    region-status) echo '{"ok":true,"country":"US","locale":"en_US.UTF-8","keyboard":"us","locales":["en_US.UTF-8"],"keyboards":["us"]}' ;;
+    region-set)    jq -cn --arg i "$input" '{ok:true,echo:$i,restart:true}' ;;
+    *)             echo '{"ok":false,"error":"unknown"}' ;;
+esac