refactor: rename projects to opensource-projects

Frees /products/ for the product page and makes the open-source archive
explicit, rather than leaving two sections that both look like "projects".

- Move the directory in both locales and update the 18 internal links
  pointing into it.
- Rework helix for the GitHub layout: server/ and web/ are directories in
  a single repository there, not the two separate repositories Gitea had.
  The "fork both repositories" step and the section wording follow.
- Correct helix's stated Java version to 21, which is what the build
  actually targets.

docs/zh-hans/opensource-projects/vite-plugins/index.mdx

@@ -0,0 +1,11 @@
+---
+title: Vite 插件
+---
+
+## 概览
+
+由 OnixByte 开发和维护的 Vite 插件合集。每个插件旨在解决 Vite 开发工作流中的特定问题 — 轻量级、零依赖、即插即用。
+
+## 可用插件
+
+- **[Port Checker](/opensource-projects/vite-plugins/port-checker)** — 当开发服务器监听浏览器限制的端口（依据 Fetch 规范）时发出警告，帮助您避免无响应的连接失败。