✨ feat(region): map timezones to regions

- ✨ one iso serves every region, so the installer derives it from the timezone the user already picks
- ✅ reject unknown zones and zones claimed by two regions

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QuATgjU4mnqpqSLAKMQ14g

default/regions/cn/timezones

@@ -0,0 +1,9 @@
+# Installs in these timezones get this region. The installer's timezone list
+# includes tzdata's backward-compatible names, so they are listed too.
+Asia/Shanghai
+Asia/Urumqi
+Asia/Chongqing
+Asia/Chungking
+Asia/Harbin
+Asia/Kashgar
+PRC