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/zh-TW.json

@@ -555,7 +555,10 @@
       "modelId": "模型 ID",
       "modelName": "模型名稱",
       "deleteModel": "刪除模型 {modelId}",
-      "nativeJsonConfig": "OpenCode 原生 JSON 配置"
+      "nativeJsonConfig": "OpenCode 原生 JSON 配置",
+      "mainModel": "主模型",
+      "smallModel": "小模型",
+      "noMatchingModels": "沒有匹配的模型"
     },
     "openClaw": {
       "gatewayConfig": "Gateway 配置",