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/en.ts

@@ -100,6 +100,8 @@ export default {
   'newSession.type.worktree.desc': 'Create a new worktree next to repo',
   'newSession.type.worktree.placeholder': 'feature-x (default 1228-xxxx)',
   'newSession.agent': 'Agent',
+  'newSession.model': 'Model',
+  'newSession.model.optional': 'optional',
   'newSession.yolo': 'YOLO mode',
   'newSession.yolo.title': 'Bypass approvals and sandbox',
   'newSession.yolo.desc': 'Uses dangerous agent flags when spawning.',