fix(frontend): stabilize diagrams and onboarding layout

[Luhaozhu]

Summary / 概述

Synchronize the latest validated frontend fixes from the FULL repository into the public CE tree.

• Add accessible zoom-out, zoom-in, and reset controls to rendered Mermaid diagrams while keeping SVG content stable across zoom changes.
• Defer Mermaid portal rendering until streaming completes and preserve the rendered HTML prop to prevent detached portal targets and repeated loading transitions.
• Keep the first-run setup shell within the viewport and make its content region independently scrollable on smaller screens.

This is an official upstream synchronization generated through the CE manifest, pruning, overlay, licensing, and brand-boundary pipeline.

Related issue / 关联 Issue

None.

Type of change / 变更类型

• 📖 Documentation (document/**, README*.md)
• 🧩 Example / sample (examples/**)
• 🔧 Repo meta (CI, templates, .github/**)
• Other (please describe / 请说明): Official upstream-derived frontend runtime and responsive-layout fixes

Checklist / 检查项

• My change only touches non-generated paths (docs / examples / repo meta). 仅改动非生成路径。
• Links and code snippets were verified. 链接与代码片段已验证。
• For bilingual docs, I updated both document/en/** and document/zh-CN/** where applicable. 双语文档已同步更新（如适用）。
• I read CONTRIBUTING.md. 我已阅读贡献指南。

The first checklist item is intentionally unchecked because this maintainer PR is an official generated-code synchronization from the upstream FULL repository. The documentation-specific checks are not applicable because no documentation changed.

Validation

• CE boundary, brand, license, backend import, pytest collection, and frontend production-build gates passed.
• The FULL frontend i18n check, TypeScript build, and Vite production build passed.
• The staged diff passed whitespace checks.
• Targeted ESLint reports one existing react-refresh/only-export-components finding on the unchanged Mermaid test-helper export; no finding points to the added lines.
• The public repository has no root package.json, so npm run preflight is not available.