fix(chat): improve agent code block contrast

[musnows]

中文

重提关闭的 PR #277。#277 的 maintainer 评论说明代码本身没有质量问题，关闭原因是当时与 #279 的 CSS 文件改动冲突。此分支已基于最新 upstream/develop 重新 cherry-pick，当前只包含同一个代码块视觉修复提交。

改动

• 为 Agent Markdown 的纯文本 fenced code block 增加独立的 ds-plain-code-block 样式类，避免目录树、纯文本片段看起来像普通正文。
• 为普通 pre、高亮代码块和纯文本代码块统一增强背景、边框和内阴影。
• 增加跨主题的 --ds-code-block-bg，让明暗主题和 ikun 主题下代码块都有明确底色。
• 补充 StreamdownCode 单测断言，确保纯文本 fenced block 保留代码块视觉类。

处理 PR #277 评论

• 已读取 fix(chat): improve agent code block contrast #277 评论：关闭原因是与 feat(workspace): add file tree preview panel #279 的 base-shell.css / markdown-code.css 改动存在冲突。
• 已同步最新 upstream/develop 到 a393b2d，并在新分支 codex/fix-agent-code-block-contrast-rebased 上重新应用修复。
• cherry-pick 无冲突，当前 diff 仍为 4 个文件、+26/-7。

验证

• npm run test -- src/renderer/src/components/chat/StreamdownCode.test.ts
• npm run typecheck
• git diff --check
• npm run dist:mac:arm64
• 解压 dist/Kun-0.1.0-mac-arm64.zip 并从 dist/Kun-0.1.0-mac-arm64-unzipped/Kun.app 启动 packaged app
• curl http://127.0.0.1:8899/health 返回 {"status":"ok","service":"kun","mode":"serve"}
• packaged app.asar 中确认包含 ds-plain-code-block 和 --ds-code-block-bg

English

Re-submission of closed PR #277. The maintainer comment on #277 stated that the change itself was fine, and the PR was closed because it conflicted with CSS changes from #279 at that time. This branch cherry-picks the same visual fix on top of the latest upstream/develop.

Changes

• Adds a dedicated ds-plain-code-block class for plain-text fenced blocks rendered by Agent Markdown, so directory trees and plain text snippets no longer blend into prose.
• Strengthens the background, border, and inset highlight for native pre, highlighted code blocks, and plain-text code blocks.
• Adds theme-level --ds-code-block-bg values so code blocks have a distinct surface across light, dark, and ikun themes.
• Extends StreamdownCode test coverage for plain-text fenced blocks.

Addressing PR #277 feedback

• Read the fix(chat): improve agent code block contrast #277 comments: it was closed because base-shell.css and markdown-code.css conflicted with feat(workspace): add file tree preview panel #279.
• Synced to latest upstream/develop at a393b2d and reapplied the fix on codex/fix-agent-code-block-contrast-rebased.
• The cherry-pick applied cleanly; the resulting diff is still 4 files, +26/-7.

Validation

• npm run test -- src/renderer/src/components/chat/StreamdownCode.test.ts
• npm run typecheck
• git diff --check
• npm run dist:mac:arm64
• Unzipped dist/Kun-0.1.0-mac-arm64.zip and launched packaged app from dist/Kun-0.1.0-mac-arm64-unzipped/Kun.app
• curl http://127.0.0.1:8899/health returned {"status":"ok","service":"kun","mode":"serve"}
• Confirmed packaged app.asar contains ds-plain-code-block and --ds-code-block-bg