fix: 在非安全上下文中为 copyMessage 添加 execCommand 备用方案

[alonguser]

在非安全上下文中（例如通过 HTTP 局域网 IP 访问），navigator.clipboard 不可用。为此，我们添加了使用 document.execCommand(‘copy’) 的备用方案，这与 ReadmeDialog.vue 和 Settings.vue 中的现有实现保持一致。

Fixes #7739

Modifications / 改动点

• 修改 dashboard/src/components/chat/MessageList.vue
• 新增 tryFallbackCopy() 函数，使用 document.execCommand('copy') 实现复制
• 修改 copyMessage()，安全上下文优先用 Clipboard API，否则回退到 tryFallbackCopy()

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

由于代码运行在 Docker 容器内，无法直接修改编译后的文件进行测试。

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Bug Fixes:

• Ensure copying chat messages works when the Clipboard API is unavailable in non-secure (e.g., HTTP) contexts by falling back to a document.execCommand-based implementation.

[gemini-code-assist]

Code Review

This pull request introduces a fallback mechanism for copying text to the clipboard using a hidden textarea when the Clipboard API is unavailable or the context is insecure. Feedback suggests making the fallback implementation more robust by using fixed positioning and read-only attributes, refactoring it into a shared utility to eliminate code duplication, and adding error handling to the primary Clipboard API call to ensure the fallback is triggered upon failure.

[gemini-code-assist]

建议优化 tryFallbackCopy 的实现并考虑将其提取为公共工具函数。设置 readOnly 可以防止在移动设备上弹出虚拟键盘，使用 position: fixed 并将其移出可视区域比 position: absolute 更稳健，能有效避免页面滚动或布局抖动。此外，PR 描述提到此逻辑在 ReadmeDialog.vue 和 Settings.vue 中也有重复，建议根据通用规则将其重构为共享函数以提高可维护性。

function tryFallbackCopy(text: string) {
  const textArea = document.createElement("textarea");
  textArea.value = text;
  textArea.readOnly = true;
  Object.assign(textArea.style, {
    position: "fixed",
    left: "-9999px",
    top: "0",
  });
  document.body.appendChild(textArea);
  textArea.select();
  try {
    document.execCommand("copy");
  } catch (err) {
    console.error("Fallback copy failed:", err);
  } finally {
    document.body.removeChild(textArea);
  }
}

References

1. 当在不同场景下实现类似功能时，应将逻辑重构为共享的辅助函数以避免代码重复。

[gemini-code-assist]

即使在安全上下文中，navigator.clipboard.writeText 也可能因为用户拒绝权限或其他运行时错误而失败。建议增加错误处理，确保在 Clipboard API 抛出异常时也能回退到 tryFallbackCopy。

  if (navigator.clipboard && window.isSecureContext) {
    try {
      await navigator.clipboard.writeText(text);
      return;
    } catch (err) {
      console.warn("Clipboard API failed, falling back:", err);
    }
  }
  tryFallbackCopy(text);

[sourcery-ai]

Hey - I've left some high level feedback:

• The new tryFallbackCopy implementation duplicates the existing clipboard fallback logic used in ReadmeDialog.vue and Settings.vue; consider extracting a shared utility to avoid divergence between implementations.
• document.execCommand('copy') is deprecated and can fail silently; it may be worth wrapping tryFallbackCopy in a try/catch and optionally checking the boolean return value so the caller can handle failures more gracefully.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `tryFallbackCopy` implementation duplicates the existing clipboard fallback logic used in `ReadmeDialog.vue` and `Settings.vue`; consider extracting a shared utility to avoid divergence between implementations.
- `document.execCommand('copy')` is deprecated and can fail silently; it may be worth wrapping `tryFallbackCopy` in a try/catch and optionally checking the boolean return value so the caller can handle failures more gracefully.

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[RC-CHN]

请注意添加测试截图，不包含测试的pr将不会被通过，本次暂且由我帮你测试，注意到该pr只能修复对于自行实现的复制部分在http连接情况下的fallback，对于你在issue中提到的情况，代码块由markstream-vue渲染，该pr无法处理在非安全连接情况下代码块的复制，请注意。

[RC-CHN]

再度进行测试后注意到document.execCommand特性已被相当大一部分浏览器弃用 需要考虑最终fallback到框选区域由用户手动ctrl+c复制的方法

[RC-CHN]

已在各个页面中测试处理

[alonguser]

感谢完善，学到了很多！