Display bandwidth stats in GTK GUI (blocked by #26)

[TheZupZup]

Overview

Once get_bandwidth_stats() exists in monitor.py (#26), this issue adds a display for it in the GTK GUI.

This issue is blocked by #26. Do not start it until get_bandwidth_stats() is implemented and merged.

What needs to be done

• Add a label or status bar at the bottom of the GTK window in main.py showing "↑ X KB/s ↓ Y KB/s"
• Call get_bandwidth_stats() inside the existing refresh_connections() method and update the label
• Format the values clearly: use KB/s for values under 1024, MB/s above
• Test at different window sizes to ensure the label doesn't cause layout issues

Goal

The GTK GUI shows current system-wide upload/download rates, updated every 2 seconds alongside the connection list.

Notes

• Blocked by Implement basic system-wide bandwidth sampling in monitor.py #26 — the get_bandwidth_stats() function must exist first
• The 2-second GLib.timeout_add_seconds refresh cycle in main.py already handles the update timing