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/de.json

@@ -555,7 +555,10 @@
       "modelId": "Modell-ID",
       "modelName": "Modellname",
       "deleteModel": "Modell {modelId} löschen",
-      "nativeJsonConfig": "OpenCode Native JSON-Konfiguration"
+      "nativeJsonConfig": "OpenCode Native JSON-Konfiguration",
+      "mainModel": "Hauptmodell",
+      "smallModel": "Kleines Modell",
+      "noMatchingModels": "Keine passenden Modelle"
     },
     "openClaw": {
       "gatewayConfig": "Gateway-Konfiguration",