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/en.json

@@ -555,7 +555,10 @@
       "modelId": "Model ID",
       "modelName": "Model Name",
       "deleteModel": "Delete model {modelId}",
-      "nativeJsonConfig": "OpenCode Native JSON Config"
+      "nativeJsonConfig": "OpenCode Native JSON Config",
+      "mainModel": "Main Model",
+      "smallModel": "Small Model",
+      "noMatchingModels": "No matching models"
     },
     "openClaw": {
       "gatewayConfig": "Gateway Config",