fix: add X-Deepin-Singleton flag to desktop file (#392841)

[add-uos]

Add X-Deepin-Singleton=true to desktop file to mark the application as a singleton app. This allows AM to identify singleton apps and inform Treeland to skip the splash screen for these apps.

Refs: https://pms.uniontech.com/task-view-392841.html

Summary by Sourcery

Enhancements:

• Add X-Deepin-Singleton=true flag to the deepin-font-manager desktop file to identify it as a singleton app.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds the X-Deepin-Singleton flag to the font manager desktop entry so the application is treated as a singleton by the application manager and Treeland (for splash behavior).

File-Level Changes

Change	Details	Files
Mark the deepin font manager desktop entry as a singleton application via a new desktop-file key.	Add X-Deepin-Singleton=true key to the desktop entry to indicate singleton behavior to the application manager Ensure the flag is placed alongside other desktop metadata keys to be recognized by Deepin/AM/Treeland tooling	deepin-font-manager-assets/deepin-font-manager.desktop

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[sourcery-ai]

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[deepin-ci-robot]

deepin pr auto review

★ 总体评分：95分

■ 【总体评价】

代码通过添加自定义配置项实现了应用单例模式，逻辑清晰且符合规范。
修改简单有效，无任何安全风险，提升了用户体验。

■ 【详细分析】

• 1.语法逻辑（完全正确）✓

修改在 deepin-font-manager-assets/deepin-font-manager.desktop 文件中新增了一行 X-Deepin-Singleton=true，符合 Desktop Entry Specification 的键值对语法规范，赋值类型正确。
潜在问题：无
建议：无需修改

• 2.代码质量（符合规范）✓

代码使用了 Deepin/UOS 桌面环境约定的扩展属性 X-Deepin-Singleton，配置项放置在 X-Deepin-Vendor 之后，排版整洁，遵循了该项目的配置文件编写习惯。
潜在问题：无
建议：无需修改

• 3.代码性能（无性能问题）✓

桌面配置文件的变更对系统启动和运行性能无负面影响，反而通过单例模式阻止应用多开，减少了不必要的多进程内存和CPU资源消耗。
潜在问题：无
建议：无需修改

• 4.代码安全（存在0个安全漏洞）✓

漏洞对比统计：新增漏洞 0 个，减少漏洞 0 个，持平 0 个
本次修改仅为桌面配置项的添加，不涉及任何可执行代码、网络请求、路径解析或文件系统操作，不存在任何安全攻击面。

• 建议：无需修改

■ 【改进建议代码示例】

StartupNotify=false
TryExec=deepin-font-manager
Type=Application
X-Deepin-Vendor=deepin
X-Deepin-Singleton=true

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: add-uos, lzwind

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• debian/deepin/OWNERS

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