feat(webui): support images and links in README dialog

[clown145]

修复 README 弹窗中相对路径图片无法显示的问题，同时支持仓库内相对链接的跳转。

当 README 中使用相对路径引用图片（如 ./images/screenshot.png）或其他文档（如 ./docs/usage.md）时，由于路径解析问题导致无法正确加载。本 PR 实现了路径自动转换功能，并复用用户已配置的 GitHub 加速代理。

Modifications / 改动点

修改文件: dashboard/src/components/shared/ReadmeDialog.vue

功能实现:

1. 相对图片路径转换为 GitHub Raw URL（https://raw.githubusercontent.com/...），支持代理加速
2. 相对文档链接转换为 GitHub Blob URL（https://github.com/.../blob/main/...）
3. 复用用户在设置中配置的 GitHub 代理（localStorage.selectedGitHubProxy）

• 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

验证步骤:

1. 运行 npm run build 通过 ✅
2. 安装包含相对路径图片的插件，查看 README 弹窗
3. 确认图片正确加载，点击仓库内文档链接可正常跳转到 GitHub

---

Checklist / 检查清单

• 😊 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。/ If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
• 👀 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"。/ My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
• 🤓 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了 requirements.txt 和 pyproject.toml 文件相应位置。/ 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.
• 😮 我的更改没有引入恶意代码。/ My changes do not introduce malicious code.

Summary by Sourcery

增强 README 对话框，使其能够正确解析并打开针对托管在 GitHub 上的插件的仓库相对链接和图片。

新功能：

• 将 README 对话框中的相对图片源转换为 GitHub 原始文件（raw）URL，并可选择通过用户配置的 GitHub 代理进行访问。
• 将 README 对话框中的仓库相对文档链接转换为 GitHub blob URL，并在新标签页中安全打开。

Original summary in English

Summary by Sourcery

Enhance the README dialog to correctly resolve and open repository-relative links and images for GitHub-hosted plugins.

New Features:

• Convert relative image sources in the README dialog to GitHub raw URLs, optionally routed through the user-configured GitHub proxy.
• Convert repository-relative document links in the README dialog to GitHub blob URLs and open them safely in a new tab.

[sourcery-ai]

Hey - 我发现了 1 个问题，并给出了一些整体性的反馈：

• 当前对 GitHub 仓库的解析和 URL 生成假定为简单的 https://github.com/owner/repo 形式，并且默认分支为 main；建议支持更多常见变体，例如 /owner/repo/、/owner/repo.git，或带有 /tree/<branch> 的 URL，这样在默认分支不是 main 的仓库中，链接和图片也能正常工作。
• parseGitHubRepoInfo 使用了相对宽松的正则表达式，可能会错误解析包含点号或额外路径段的仓库名；如果收紧这部分逻辑（例如显式去掉 .git 并忽略尾部额外路径），可以让 URL 转换更加健壮。

给 AI 代理的提示

Please address the comments from this code review:

## Overall Comments
- 当前对 GitHub 仓库的解析和 URL 生成假定为简单的 `https://github.com/owner/repo` 形式，并且默认分支为 `main`；建议支持更多常见变体，例如 `/owner/repo/`、`/owner/repo.git`，或带有 `/tree/<branch>` 的 URL，这样在默认分支不是 `main` 的仓库中，链接和图片也能正常工作。
- `parseGitHubRepoInfo` 使用了相对宽松的正则表达式，可能会错误解析包含点号或额外路径段的仓库名；如果收紧这部分逻辑（例如显式去掉 `.git` 并忽略尾部额外路径），可以让 URL 转换更加健壮。

## Individual Comments

### Comment 1
<location> `dashboard/src/components/shared/ReadmeDialog.vue:57-60` </location>
<code_context>
 });

+// 解析 GitHub 仓库信息
+function parseGitHubRepoInfo(repoUrl) {
+  if (!repoUrl) return null;
+  // 支持格式: https://github.com/user/repo 或 https://github.com/user/repo.git
+  const match = repoUrl.match(/github\.com\/([^/]+)\/([^/.]+)/);
+  if (!match) return null;
+  return { owner: match[1], repo: match[2] };
</code_context>

<issue_to_address>
**issue (bug_risk):** 用于解析 GitHub 仓库的正则会漏掉包含点号的合法仓库名，并且可能错误处理 `.git` 后缀。

`([^/.]+)` 会在遇到 `.` 或 `/` 时停止，因此像 `my.repo` 这样的仓库名会被解析为 `my`。它之所以“去掉” `.git`，只是因为在 `.` 处停止，从而将仓库名中的合法点号与 `.git` 后缀混在一起处理。更清晰的做法是捕获直到下一个 `/` 的所有内容，然后再去掉可选的 `.git` 后缀：

```js
const match = repoUrl.match(/github\.com\/([^/]+)\/([^/]+)(?:\.git)?/);
if (!match) return null;
const owner = match[1];
const repo = match[2].replace(/\.git$/, "");
return { owner, repo };
```

这样可以正确处理 `user/repo`、`user/repo.git`，以及名称中包含点号的仓库。
</issue_to_address>

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Original comment in English

Hey - I've found 1 issue, and left some high level feedback:

• The GitHub repo parsing and URL generation assumes a simple https://github.com/owner/repo shape and a main default branch; consider handling more common variants such as /owner/repo/, /owner/repo.git, or URLs with /tree/<branch> so links and images work for non-main default branches.
• parseGitHubRepoInfo uses a relatively loose regex that may mis-parse repository names containing dots or additional path segments; tightening this logic (e.g., explicitly stripping .git and ignoring trailing paths) would make the URL conversions more robust.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The GitHub repo parsing and URL generation assumes a simple `https://github.com/owner/repo` shape and a `main` default branch; consider handling more common variants such as `/owner/repo/`, `/owner/repo.git`, or URLs with `/tree/<branch>` so links and images work for non-`main` default branches.
- `parseGitHubRepoInfo` uses a relatively loose regex that may mis-parse repository names containing dots or additional path segments; tightening this logic (e.g., explicitly stripping `.git` and ignoring trailing paths) would make the URL conversions more robust.

## Individual Comments

### Comment 1
<location> `dashboard/src/components/shared/ReadmeDialog.vue:57-60` </location>
<code_context>
 });

+// 解析 GitHub 仓库信息
+function parseGitHubRepoInfo(repoUrl) {
+  if (!repoUrl) return null;
+  // 支持格式: https://github.com/user/repo 或 https://github.com/user/repo.git
+  const match = repoUrl.match(/github\.com\/([^/]+)\/([^/.]+)/);
+  if (!match) return null;
+  return { owner: match[1], repo: match[2] };
</code_context>

<issue_to_address>
**issue (bug_risk):** Regex for parsing GitHub repo misses valid repo names containing dots and may mis-handle `.git` suffix.

`([^/.]+)` stops at `.` or `/`, so repos like `my.repo` are parsed as `my`. It also only strips `.git` because of the `.` stop, mixing up valid dots in repo names with the `.git` suffix. A clearer approach is to capture everything to the next `/` and then strip an optional `.git` suffix:

```js
const match = repoUrl.match(/github\.com\/([^/]+)\/([^/]+)(?:\.git)?/);
if (!match) return null;
const owner = match[1];
const repo = match[2].replace(/\.git$/, "");
return { owner, repo };
```

This correctly handles `user/repo`, `user/repo.git`, and repos with dots in the name.
</issue_to_address>

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