feat(adapters): mark EBA-superseded adapters as legacy and collapse them

The 12 old adapters that now have an EBA replacement are tagged
`spec.legacy: true` in their source manifests. Principle: don't delete,
de-emphasize.

- sources/*.yaml (aiocqhttp, dingtalk, discord, kook, lark,
  officialaccount, qqofficial, slack, telegram, wecom, wecombot,
  wecomcs): add spec.legacy: true
- Adapter / IChooseAdapterEntity types: add optional legacy flag
- BotForm adapter Select: split legacy adapters into a collapsed,
  grayscale group at the bottom with an explanatory hint; auto-expand
  when the bot already uses a legacy adapter
- Wizard platform picker: same collapsed legacy section
- i18n: legacyAdapters / legacyAdaptersHint (zh-Hans, en-US)

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

src/langbot/pkg/platform/sources/slack.yaml

@@ -20,6 +20,7 @@ metadata:
     es_ES: Adaptador de Slack, requiere una dirección pública para recibir notificaciones de mensajes de Slack, consulte la documentación para obtener instrucciones de uso
   icon: slack.png
 spec:
+  legacy: true
   categories:
     - popular
     - global