fix(ui): restore Mermaid fullscreen wheel zoom

[loto585]

Linked issue

Closes #522

Summary

修复 Mermaid 全屏模式下无法通过鼠标滚轮缩放的问题。

PR #535 将 Mermaid 全屏实现切换为共享 Dialog 后，滚轮监听仍然绑定在全屏视口的冒泡阶段。SVG 子元素或弹层事件传播可能在事件到达监听器前将其截断，导致按钮缩放正常，但鼠标滚轮缩放失效。

本次修改将滚轮处理移动到全屏视口的捕获阶段，并阻止事件继续传播到外层 Dialog，继续使用 SVG viewBox 进行无损缩放，不会重新引入缩放模糊问题。

Change scope

• Modules: agent-ui
• Key paths:
  • crates/agent-ui/src/components/MarkdownMermaidFullscreen.tsx

Screenshots / preview

Verification

• node --test crates/agent-gui/test/chat/mermaid-viewbox.test.mjs
• pnpm build:gui
• pnpm build:webui
• pnpm --filter @liveagent/ui exec biome check src/components/MarkdownMermaidFullscreen.tsx
• git diff --check upstream/main...HEAD

Behavior

• 鼠标滚轮向上：放大 Mermaid
• 鼠标滚轮向下：缩小 Mermaid
• 放大、缩小和重置按钮保持正常
• 缩放继续通过 SVG viewBox 完成，不使用 CSS 位图缩放

Pre-submit checklist

• A requirement issue is linked (or this is a trivial fix that needs no issue, as explained in the summary).
• Synced with the target branch; no merge conflicts.
• The change is focused, with no unrelated modifications.
• No secrets, tokens, or personal data included.
• Docs are updated for changes affecting user behavior, deployment, or configuration.(改动未修改操作方式和配置, 无需更新文档)