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/hooks/mutations/useSpawnSession.ts

@@ -7,6 +7,7 @@ type SpawnInput = {
     machineId: string
     directory: string
     agent?: 'claude' | 'codex' | 'gemini'
+    model?: string
     yolo?: boolean
     sessionType?: 'simple' | 'worktree'
     worktreeName?: string
@@ -28,6 +29,7 @@ export function useSpawnSession(api: ApiClient | null): {
                 input.machineId,
                 input.directory,
                 input.agent,
+                input.model,
                 input.yolo,
                 input.sessionType,
                 input.worktreeName