feat: add model selection for AI agents (claude, codex, gemini)

Add a new ModelSelector component that allows users to choose specific model variants for different AI agents. This includes:
- ModelSelector UI component with agent-specific model options
- Model parameter propagation through API, RPC, and spawning layers
- Support for auto model selection (default behavior)
- Localization strings for English and Chinese
- Type definitions for model options per agent

web/src/lib/locales/zh-CN.ts

@@ -102,6 +102,8 @@ export default {
   'newSession.type.worktree.desc': '在仓库旁创建新工作树',
   'newSession.type.worktree.placeholder': 'feature-x (默认 1228-xxxx)',
   'newSession.agent': '代理',
+  'newSession.model': '模型',
+  'newSession.model.optional': '可选',
   'newSession.yolo': 'YOLO 模式',
   'newSession.yolo.title': '跳过审批和沙箱',
   'newSession.yolo.desc': '启动时使用危险的代理标志。',