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/ja.json

@@ -555,7 +555,10 @@
       "modelId": "モデル ID",
       "modelName": "モデル名",
       "deleteModel": "モデル {modelId} を削除",
-      "nativeJsonConfig": "OpenCode ネイティブ JSON 設定"
+      "nativeJsonConfig": "OpenCode ネイティブ JSON 設定",
+      "mainModel": "メインモデル",
+      "smallModel": "スモールモデル",
+      "noMatchingModels": "一致するモデルがありません"
     },
     "openClaw": {
       "gatewayConfig": "Gateway 設定",