Allow text selection inside code blocks

[dcdyouget]

Summary

• Register syntax-highlighted code lines with the shared Markdown selection system.
• Support partial, multi-line, and prose-to-code selections in transcripts and Markdown Preview.
• Preserve blank code lines when copying a selection across them.
• Keep syntax highlighting, horizontal scrolling, Fit Content, and whole-block Copy behavior unchanged.

This is a focused extraction and adaptation of the code-selection work from the closed #331. It intentionally excludes the unrelated settings and task-group changes from that PR.

Fixes #332

中文说明

问题

聊天记录和 Markdown 预览中的代码块只能通过右上角的 Copy 按钮复制全部内容，无法像普通 Markdown 文本一样拖选并复制其中某一行或部分代码，也无法从普通文本连续选择到代码块内容。

修复内容

• 将带语法高亮的每一行代码接入现有 Markdown 文本选择系统。
• 支持单行、跨多行以及普通文本与代码块之间的连续选择。
• 跨越空白代码行时保留原有换行，避免复制结果丢失空行。
• 保持语法高亮、横向滚动、Fit Content 和整块复制按钮的原有行为不变。

本 PR 从已关闭的 #331 中提取并适配代码块选择相关实现，未包含其中无关的设置和任务分组改动；用于修复 #332。

Testing

• cargo test -p zeron-ui --lib (960 passed)
• rustfmt --edition 2024 --check crates/ui/src/markdown/render.rs crates/ui/src/markdown/selection.rs
• git diff --check

[wingleeio]

Security audit of 65fda35: approved. No malicious code or new merge-blocking security issue found in the reviewed changes.

Reviewed the complete commit and two-file diff, plus surrounding Markdown selection, text-layout geometry, preview scopes, clipboard actions, code rendering/caching, and transcript selection behavior. The production change registers syntax-highlighted source lines with the existing selection machinery and preserves interior empty lines when joining selected spans. Rendering and copy use the same source text; no code evaluation, clipboard substitution, network access, credential handling, file access, new dependency, CI modification, binary asset, executable mode or symlink is introduced. New code-line keys preserve the Markdown Preview scope prefix. Selection listeners and Linux primary-selection behavior reuse the existing user-gesture-driven path.

Validation:

• Combined the PR with current main 9601a24 in an isolated worktree. The resulting local merge ff3b3ec64f5c8c1e98cba10e7688ac8e1a7dd368 differs from main only in the two intended Markdown files.
• cargo test --locked -p zeron-ui --lib -- --test-threads=1: 1001 passed, zero failures, including code-line selection, blank-line preservation, preview isolation, scrolling, copy and Fit Content regressions.
• An independent harness importing the exact production selection.rs passed 1444 valid-character-boundary Unicode/empty-line/reverse-direction cases. This is additional coverage, not exhaustive fuzzing or a GPUI mouse-geometry test.
• rustfmt checks and git diff --check passed; added-line hidden/control-character scan found none.
• Existing head CI is green for desktop UI, session synchronization, iOS, Linux browser and macOS native/frame recovery. Those remote runs predate the latest main merge; current-main compatibility was tested locally as described above.

The assessment is scoped to this PR and its affected paths; source review cannot prove contributor intent or guarantee zero vulnerabilities. No new manual native-platform test was performed beyond existing CI and the local headless desktop suite.