feat(config): add Lark/Feishu channel config support

* feat(config): add Lark/Feishu channel config support

- Add LarkConfig struct with app_id, app_secret, encrypt_key, verification_token, allowed_users, use_feishu fields
- Add lark field to ChannelsConfig
- Export LarkConfig in config/mod.rs
- Add 5 tests for LarkConfig serialization/deserialization

Related to #164

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: apply cargo fmt formatting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/onboard/wizard.rs

@@ -1126,6 +1126,7 @@ fn setup_channels() -> Result<ChannelsConfig> {
         whatsapp: None,
         email: None,
         irc: None,
+        lark: None,
     };
 
     loop {