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/index.md

@@ -0,0 +1,25 @@
+---
+title: Open Source
+---
+
+Open-source projects developed by OnixByte.
+
+## Libraries & Toolkits
+
+- **[OnixByte Toolbox](/opensource-projects/onixbyte-toolbox)** — A development kit for Java applications: AES, hashing, Base64, collection utilities, crypto key loaders, statistical calculators, identity generators, and generic tuples.
+- **[regions4j](/opensource-projects/regions4j)** — Type-safe ISO 3166-1 regional metadata as a Java enum with O(1) lookups, E.164 dialling codes, and locale tags.
+- **[Captcha](/opensource-projects/captcha)** — A modern Java CAPTCHA generation library with water ripple, fish-eye, and shadow distortion effects.
+- **[Calendar Toolbox](/opensource-projects/calendar-toolbox)** — A Java library for creating `.ics` files conforming to the iCalendar specification (RFC 5545).
+
+## Full-Stack Applications
+
+- **[Helix](/opensource-projects/helix)** — An enterprise application template with Spring Boot backend and React frontend, featuring JWT auth, RBAC, user and organisation management, and Microsoft Entra ID integration.
+- **[Delta Force Guide](/opensource-projects/delta-force-guide)** — A web app providing a searchable library of firearm modification codes for the game _Delta Force: Havvk Ops_, with a React SPA and Spring Boot REST API.
+
+## Developer Tools
+
+- **[Claude Code Skills Manager](/opensource-projects/claude-code-skill-manager)** — A CLI tool for discovering, installing, and managing Claude Code skills from Git-based marketplace repositories.
+
+## Vite Plugins
+
+- **[Port Checker](/opensource-projects/vite-plugins/port-checker)** — Warns when the Vite dev server listens on a browser-restricted port, preventing silent connection failures.