Surface blocking errors to the user (after iptables backend exists)

[TheZupZup]

Overview

Currently, the only blocking operation is writing to the rules file, which already handles errors (though silently — see #10). This issue becomes meaningful once real OS-level blocking (#20) is implemented via iptables, where failures are much more likely (insufficient permissions, iptables not installed, rule already exists).

This issue is blocked by #20. Do not start it until the iptables backend is merged.

What needs to be done

• After Implement iptables backend for OS-level IP blocking #20 is merged: wrap apply_block() and remove_block() calls in both tui.py and main.py with visible error feedback
  • TUI: use self.notify() for transient error messages
  • GTK GUI: use the existing show_error() dialog method
• Distinguish between at least two failure modes:
  • iptables not found on the system → "iptables is not available on this system"
  • Permission denied → "Run SilentGuard with elevated privileges to enable OS-level blocking"
• If iptables fails but the rules-file update succeeded, tell the user: "IP marked as blocked in SilentGuard but OS-level blocking failed"

Goal

When a block action fails at the OS level, the user sees a specific and actionable error message instead of a silent failure or a misleading success indicator.

Notes

Blocked by #20 — do not start this until apply_block() / remove_block() exist.