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/api/client.ts

@@ -352,13 +352,14 @@ export class ApiClient {
         machineId: string,
         directory: string,
         agent?: 'claude' | 'codex' | 'gemini',
+        model?: string,
         yolo?: boolean,
         sessionType?: 'simple' | 'worktree',
         worktreeName?: string
     ): Promise<SpawnResponse> {
         return await this.request<SpawnResponse>(`/api/machines/${encodeURIComponent(machineId)}/spawn`, {
             method: 'POST',
-            body: JSON.stringify({ directory, agent, yolo, sessionType, worktreeName })
+            body: JSON.stringify({ directory, agent, model, yolo, sessionType, worktreeName })
         })
     }