Warn when adding a domain that is already blocked

[TwDover]

Summary

• Adds an is_domain_blocked() helper that walks the GTK tree model to check if a domain is already in the list
• Before inserting, checks all variants that would be added (e.g. both example.com and www.example.com)
• Shows an error dialog with a clear message if a duplicate is detected, preventing the same domain being written to /etc/hosts multiple times

Test plan

• Add a domain (e.g. example.com) — should succeed as normal
• Try adding the same domain again — should show "Already Blocked" error dialog instead of adding a duplicate
• Try adding www.example.com after example.com is blocked — should also show the error
• Remove the domain and re-add it — should succeed again

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