feat(settings): add model selection combobox for OpenCode agent configuration

Replace plain text inputs for model and small_model with searchable
combobox dropdowns that list models from configured providers, while
still supporting custom text entry on blur.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/i18n/messages/ko.json

@@ -555,7 +555,10 @@
       "modelId": "모델 ID",
       "modelName": "모델 이름",
       "deleteModel": "모델 {modelId} 삭제",
-      "nativeJsonConfig": "OpenCode 네이티브 JSON 구성"
+      "nativeJsonConfig": "OpenCode 네이티브 JSON 구성",
+      "mainModel": "메인 모델",
+      "smallModel": "스몰 모델",
+      "noMatchingModels": "일치하는 모델 없음"
     },
     "openClaw": {
       "gatewayConfig": "Gateway 구성",