feat(agent): support image avatars alongside emoji avatars

[leedalei]

Summary

This PR adds managed image avatar support for custom agents while preserving the existing emoji-based workflow.

这次改动把 Agent 头像从只能使用 Emoji，升级为同时支持 Emoji 和图片，但尽量不改变用户原有的使用习惯。对轻量场景，用户依然可以快速选一个 Emoji 完成设置；对更强调辨识度和角色感的场景，也可以直接上传图片。这样既提升了 Agent 的个性化表达能力，也增强了侧边栏、列表和设置页中的识别效率，让多 Agent 场景下的区分更直观，整体体验更完整、更自然，也更接近真实产品可长期使用的状态。

What changed

• add managed avatar file import, validation, and cleanup in the main process
• support avatar image persistence on agent create, update, and delete flows
• expose file selection and local preview support through preload and renderer typings
• introduce reusable avatar rendering and editing components for image + emoji avatars
• update create/settings/sidebar/agent list UI to display uploaded avatars
• keep emoji avatars available as a lightweight option alongside image avatars
• add localized error handling for unsupported types, file size limits, and import failures
• add tests for avatar storage behavior and related persistence updates

Testing

Verified:

• npx eslint src/renderer/components/agent/AgentCreateModal.tsx
• npx tsc --noEmit --project tsconfig.json

Additional coverage added:

• src/main/agentAvatarStore.test.ts

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