introduce AddOns ecosystem with APRS and Antenna tools (Userscripts)

[frankenstein91]

What does this PR do?

Idea thanks to: #465 (comment)

This PR introduces a dedicated AddOns ecosystem for OpenHamClock via Userscripts (Tampermonkey/Greasemonkey). It allows users to add specialized, optional features without increasing the core bundle size or forcing API configurations on every user.

Key additions:

• HFJ-350M Calculator: Antenna tool for the Comet multi-band antenna (DE/EN/JA).
• APRS Newsfeed (Inbox): Integrated message center for incoming APRS messages via aprs.fi.
• APRS Auto-Position: Live coordinates update for mobile/portable users based on APRS beacons.
• 🧩 Shared AddOn Drawer: A non-intrusive launcher system in the bottom right corner that reveals tools on click and allows layout rotation via right-click.
• CI Workflow Optimization: Updated .github/workflows/*.yml to skip execution on forks, reducing notification noise for community developers.

Type of change

• Bug fix
• New feature
• Performance improvement
• Refactor / code cleanup
• Documentation
• Translation
• Map layer plugin

How to test

1. Install a Userscript Manager (e.g., Tampermonkey).
2. Load any script from the AddOns/ directory (e.g., AddOns/HFJ350M-Calculator/hfj350m_calculator.user.js).
3. Open OpenHamClock and look for the 🧩 icon in the bottom right corner.
4. Click to expand tools, and right-click to switch between horizontal/vertical layouts.
5. Verify that the tools adapt to your current theme (Dark, Light, Retro) and language.

Checklist

• App loads without console errors
• Tested in Dark, Light, and Retro themes
• Responsive at different screen sizes (desktop + mobile)
• If touching server.js: N/A (Scripts are frontend-only)
• If adding an API route: N/A
• If adding a panel: Implemented as modular Userscript overlays to keep layouts clean.
• No hardcoded colors — uses CSS variables (var(--accent-cyan), etc.)
• No .bak, .old, console.log debug lines, or test scripts included

Screenshots (if visual change)

[frankenstein91]

as requested in #552 (comment)