feat(groups): configure message language per group instead of per route

Route.lang is removed; Group.lang now drives the message language for
every route in the group (default en, custom via KV i18n:<lang>).
dispatch.ts loads translations per group and falls back to en for
groups without a lang. Admin UI: GroupEditor gains the language field,
RouteEditor/RouteCard drop theirs; docs and config example updated.

config.example.yaml

@@ -13,6 +13,7 @@ groups:
     # owners: ["myorg"]            # restrict events to this org/user (optional)
     # providers: ["github", "gitea"]  # restrict to source platforms (optional; empty = all)
     # emoji: true                  # include emoji in messages (default true)
+    # lang: "en"                   # message language for this group's routes (en/zh, default en)
 
 routes:
   - id: all-push