Add streaming support and refactor Chat UI

- Add run_stream method to AgentCore for streaming output
- Add base_url parameter to LLM clients for OpenRouter support
- Add xbot module for new agent implementation
- Refactor Chat.vue into composable + components (ChatHeader, ChatMessage, ChatInput, ChatSidebar, ChatAgentSelector)
- Add ChatStream handler for SSE streaming in Go server
- Add UseXBot field to chat request

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/views/settings/useModelSettings.ts

@@ -165,6 +165,7 @@ export function useModelSettings() {
         body: JSON.stringify({
           provider: editForm.value.provider,
           model: editForm.value.model,
+          model_type: editForm.value.modelType || 'chat',
           api_key: editForm.value.apiKey,
           base_url: editForm.value.baseUrl,
           api_endpoint: editForm.value.apiEndpoint,