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/en-gb/opensource-projects/vite-plugins/index.mdx

@@ -0,0 +1,11 @@
+---
+title: Vite Plugins
+---
+
+## Overview
+
+A collection of Vite plugins developed and maintained by OnixByte. Each plugin is designed to solve a specific problem in the Vite development workflow — lightweight, zero-dependency, and plug-and-play.
+
+## Available Plugins
+
+- **[Port Checker](/opensource-projects/vite-plugins/port-checker)** — Warns when the dev server listens on a browser-restricted port (per the Fetch specification), helping you avoid silent connection failures.