feat: add webhook notifications for port events

[productdevbook]

Summary

• Adds configurable webhook URL in Settings to receive JSON POST notifications
• Supports three event types: port_opened, port_closed, port_killed
• Per-event toggle checkboxes to control which events trigger webhooks
• "Test Webhook" button to verify configuration with a test payload
• Webhooks fire on port scan changes and process kills

Payload Format

{
  "event": "port_opened",
  "port": 3000,
  "process": "node",
  "pid": 12345,
  "timestamp": "2025-01-15T10:30:00Z",
  "hostname": "MacBook-Pro.local"
}

Closes #29

Test plan

• Set a webhook URL in Settings > Webhooks
• Click "Test Webhook" — verify success indicator
• Start a process on a port — verify port_opened webhook fires
• Stop that process — verify port_closed webhook fires
• Kill a process via PortKiller — verify port_killed webhook fires
• Disable specific events — verify they no longer fire
• Clear URL — verify no webhooks are sent

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