Add autostart on WiFi connect

[AndyHazz]

Summary

• Add a "Start automatically on WiFi" toggle in the Syncthing menu
• When enabled, Syncthing starts automatically on NetworkConnected and stops on NetworkDisconnecting
• Toggling the setting on while WiFi is already connected starts Syncthing immediately
• Setting persists across restarts via G_reader_settings

Implementation

Uses KOReader's standard event system — onNetworkConnected / onNetworkDisconnecting handlers are dynamically registered/unregistered based on the setting, following the same pattern as KOSync's auto-sync.

Changes

• main.lua: added registerEvents(), event handlers, autostart toggle menu item, and G_reader_settings persistence

Test plan

• Enable "Start automatically on WiFi" → verify Syncthing starts if WiFi is already on
• Disable WiFi → verify Syncthing stops
• Re-enable WiFi → verify Syncthing starts automatically
• Disable the toggle → verify Syncthing no longer auto-starts/stops on WiFi changes
• Restart KOReader → verify the setting persists