Add trust-based color coding to GTK GUI connection list

[TheZupZup]

Overview

The TUI already color-codes connections by trust level (red for Unknown, green for Known, yellow for Local, bold red for Blocked). The GTK GUI displays the trust value in a text column but uses no colors — all rows look identical visually regardless of trust status.

What needs to be done

• For the Trust column's CellRendererText, use TreeViewColumn.set_cell_data_func() to apply colors dynamically based on the trust value of each row
• Suggested color scheme to match the TUI:
  • Unknown → red foreground (#cc0000 or red)
  • Known → green foreground (#2d8a2d or darkgreen)
  • Local → grey foreground (#888888)
  • Blocked → red foreground + bold weight
• Optionally, apply a subtle background color to the entire row for Unknown and Blocked connections for higher visibility

Goal

Trust status is visually distinct at a glance in the GTK GUI, matching the color coding the TUI already provides.

Notes

• This is purely visual — no data or logic changes needed
• The trust string value is already stored in the Gtk.ListStore model and available to set_cell_data_func()
• Example pattern from GTK docs: column.set_cell_data_func(renderer, self._color_cell, None)