feat: polish Widget Inspector tree viewer UX and make sync-docs incremental

[AmisKwok]

Summary / 摘要

Polish the Widget Inspector tree viewer UX and make the docs sync hook incremental.
优化 Widget Inspector 树查看器交互体验,并将文档同步钩子改为增量同步。

Changes / 变更

• Remove purple current-node highlight in the list (breadcrumb already marks it) / 列表移除当前节点紫色高亮(面包屑已标识)
• Move refresh button to the top bar and drop the bottom hint toolbar / 刷新按钮移到顶部栏,移除底部提示栏
• List children only (exclude the current node row) / 列表只显示子节点,不再重复当前节点
• Add varied-size demo widgets in the example app to show size changes / 示例新增多种尺寸节点以展示 size 变化
• Drop the "Children (tap to view)" section from the detail panel / 详情区移除冗余的 Children 区块
• Make sync-docs.mjs sync incrementally instead of wiping docs/ / sync-docs.mjs 改为增量同步,避免整树删除

Context / 背景

The breadcrumb already identifies the current node, so the list no longer needs a purple highlight or a duplicate current-node row. The sync-docs hook previously deleted the whole docs/ tree, which triggered bulk-delete guards; incremental sync only removes stale orphans.
面包屑已标识当前节点,列表无需紫色高亮或重复行。原 sync-docs 会清空整个 docs/ 树,触发批量删除保护;增量同步只清理过期孤儿文件。

Checklist / 检查项

• Title follows Conventional Commits / 标题符合约定式提交
• CI checks pass after merge / 合入后 CI 通过

Test plan

• flutter analyze passes (No issues found) / 分析无错误
• flutter test green on widget inspector tests / Widget Inspector 测试通过

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