fix: use lightweight endpoints with fallbacks for connectivity check

[edospadoni]

Summary

• Replace google.com with dedicated connectivity check endpoints
• Add fallback to Cloudflare endpoints if Google fails
• Use HEAD requests instead of GET to minimize traffic
• Add quick pre-check with net.isOnline() / navigator.onLine
• Reduce timeout from 5s to 3s per endpoint

Endpoints (in order)

1. connectivitycheck.gstatic.com/generate_204 (Google)
2. 1.1.1.1/cdn-cgi/trace (Cloudflare)
3. cloudflare.com/cdn-cgi/trace (Cloudflare)

Ref: NethServer/dev#7849

[github-actions]

Automatic builds from https://github.com/NethServer/nethlink/actions/runs/21705833848.
Commit: b75c3c2

Name	Platform	Link
win-app.exe	Windows (x64)	Link
macos-app-x64.dmg	MacOS (x64)	Link
macos-app-arm64.dmg	MacOS (arm64)	Link
linux-app.AppImage	Linux (x64)	Link

[github-actions]

Automatic builds from https://github.com/NethServer/nethlink/actions/runs/21706906594.
Commit: d529f42

Name	Platform	Link
win-app.exe	Windows (x64)	Link
macos-app-x64.dmg	MacOS (x64)	Link
macos-app-arm64.dmg	MacOS (arm64)	Link
linux-app.AppImage	Linux (x64)	Link

[github-actions]

Automatic builds from https://github.com/NethServer/nethlink/actions/runs/21707182079.
Commit: c20ab56

Name	Platform	Link
win-app.exe	Windows (x64)	Link
macos-app-x64.dmg	MacOS (x64)	Link
macos-app-arm64.dmg	MacOS (arm64)	Link
linux-app.AppImage	Linux (x64)	Link

[github-actions]

Automatic builds from https://github.com/NethServer/nethlink/actions/runs/21707607688.
Commit: a3cac3d

Name	Platform	Link
win-app.exe	Windows (x64)	Link
macos-app-x64.dmg	MacOS (x64)	Link
macos-app-arm64.dmg	MacOS (arm64)	Link
linux-app.AppImage	Linux (x64)	Link