Fix code block copy button positioning / 修复代码块复制按钮定位

[SivanCola]

Summary

• Move the code-block copy button out of the scrollable <pre> and keep it as a sibling inside .code-block__wrap.
• Update hover and creation-mode CSS selectors so copy-button styling follows the new sibling contract.
• Add a focused frontend regression that verifies the rendered DOM shape and guards the selector contract.

Fixes #5096.

Consolidation notes

Kept/adapted:

• fix(desktop): copy button scroll, small-window layout, settings modal… #5234 by @myipanta: adapted the core .code-block__wrap sibling approach for [Bug]: 代码块横向滚动时复制按钮会跟随内容移动 #5096. The unrelated [Feature]: 设置的弹窗建议加一个外边框或者阴影，区分与主窗口的边界 #5095 settings modal, Bug: 小窗口下界面元素错位 #5100 small-window layout, and bridge typing changes are intentionally left out of this narrow PR.
• fix(desktop): 修复代码块复制按钮跟随滚动条移动的问题 #5247 by @Neutronsoup: adapted the focused DOM move plus the creation-mode sibling selector updates.

Reviewed but not adopted:

• fix(rendering): prevent copy button from scrolling with code block content #5118 by @HUQIANTAO: validated the root-cause analysis, but did not keep the single overflow: hidden fix because it does not update the stale descendant selector contract and can mask the DOM ownership problem.
• feat(desktop): code block header toolbar / 代码块固定头部工具栏 #3120 by @HUQIANTAO: reviewed the broader code-block toolbar direction, but deferred toolbar/copy-as-Markdown behavior because this PR only fixes the pinned copy-button regression.

Repository Contributor Credits

• @myipanta contributed the multi-issue PR that included the wrapper/sibling direction for [Bug]: 代码块横向滚动时复制按钮会跟随内容移动 #5096.
• @Neutronsoup contributed the focused [Bug]: 代码块横向滚动时复制按钮会跟随内容移动 #5096 fix and creation-mode selector update.
• @HUQIANTAO contributed the earlier [Bug]: 代码块横向滚动时复制按钮会跟随内容移动 #5096 root-cause writeup and the broader code-block toolbar exploration.

Cache impact

Cache-impact: none — desktop frontend DOM/CSS/test changes only; no provider request, system prompt, tool schema, memory prefix, compaction, or reasoning upload behavior changes.
Cache-guard: N/A for cache; frontend regression and CSS checks cover this UI contract.
System-prompt-review: N/A.

Verification

• pnpm --dir desktop/frontend exec tsx src/__tests__/code-block-copy-position.test.tsx
• pnpm --dir desktop/frontend check:css
• pnpm --dir desktop/frontend typecheck
• git diff --check
• Browser smoke against the Vite dev server: injected a long code-block fixture and verified the copy button is a sibling of the scrollable <pre>, the <pre> still scrolls, the creation-mode sibling selector matches, and the old descendant selector does not match.