Add network RX/TX metrics to TUI top-line header

[Copilot]

The issue requested new top-line metrics in the main TUI header. This update adds cumulative network receive/transmit badges so header telemetry includes memory, swap, load, and network activity.

• Header metrics update (ptop3/monitor.py)

  • Added network counters to TUI.draw() via psutil.net_io_counters().
  • Rendered two new badges in the existing top-line badge row:
    • rx = bytes_recv in GiB
    • tx = bytes_sent in GiB
  • Added safe fallback (0.0) when network counters are unavailable.

• Targeted monitor test updates (tests/test_monitor.py)

  • Mocked psutil.net_io_counters() in draw-path tests.
  • Asserted that RX/TX labels and values appear in rendered header output.
  • Preserved existing coverage expectations by widening one fake screen scenario so full header badges render.

• User-facing docs/changelog

  • Updated README feature summary to include network badges in the colored header.
  • Added an Unreleased changelog entry describing the new RX/TX top-line metrics.

net = psutil.net_io_counters()
rx_gib = net.bytes_recv / (1024 ** 3)
tx_gib = net.bytes_sent / (1024 ** 3)

badges.extend([
    ("rx", f"{rx_gib:.1f}G", bg_label, bg_neut),
    ("tx", f"{tx_gib:.1f}G", bg_label, bg_neut),
])