config: Default language to English (en)

Change the default app language from zh-CN to en so that the i18n-seeded
default group names ("Default Group"/"Share Group" instead of 默认组/共享组)
and all backend i18n responses default to English.

Note: existing databases already seeded under zh-CN keep their Chinese
group names and must be renamed manually.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

conf/config.yaml

@@ -1,4 +1,4 @@
-lang: "zh-CN"
+lang: "en"
 app:
   web-client: 1  # 1:enable 0:disable
   register: false #whether registration is enabled