fix: address review findings from security and architecture audit

[GeiserX]

Summary

Fixes from a multi-agent review (code, security, architecture, test coverage):

• Hosts collision detection: now handles tab-separated entries, multi-hostname lines, and inline comments — previously only split on spaces, missing standard /etc/hosts format
• Quit timeout safety net: 8-second hard deadline ensures termination even if cleanup Task stalls beyond XPC timeouts
• Proxy server validation: rejects flag injection (e.g. -e /bin/sh) before passing to /usr/bin/nc
• DNS domain validation: rejects domains starting with - before passing to dig/kdig/curl

Not addressed (separate work)

• XPC code-signing anchor check (needs Developer ID cert info)
• Proxy password to Keychain (significant refactor)
• Log file from /tmp to ~/Library/Application Support

Test plan

• Verify VPN Bypass skips hosts entries managed by other tools (tab-separated, multi-hostname)
• Quit the app and verify it exits cleanly within seconds
• Enter invalid proxy server values and verify they are rejected
• Verify normal DNS resolution still works for configured domains

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b4535d94-59de-448e-b421-2387caeb69fe

📥 Commits

Reviewing files that changed from the base of the PR and between 2b85979 and eefe42c.

📒 Files selected for processing (3)

• Helper/HelperTool.swift
• Sources/RouteManager.swift
• Sources/VPNBypassApp.swift

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/review-findings

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