Display bandwidth stats in TUI (blocked by #26)

[TheZupZup]

Overview

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

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 footer widget or extend the existing status bar in tui.py to show "↑ X KB/s ↓ Y KB/s"
• Call get_bandwidth_stats() in the existing refresh handler and update the widget
• Format the values clearly: KB/s below 1024, MB/s above
• Verify the layout doesn't break on small terminal sizes (80x24 minimum)

Goal

The TUI shows current system-wide upload/download rates, updated each refresh cycle, without breaking the existing layout.

Notes

• Blocked by Implement basic system-wide bandwidth sampling in monitor.py #26 — the get_bandwidth_stats() function must exist first
• Textual's Footer widget or a Static widget in the layout are both reasonable approaches