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:

• NethLink: connectivity check causes rate limiting (429) on startup with many clients dev#7849

[github-actions]

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[github-actions]

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[github-actions]

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[github-actions]

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