Update auction email templates and ban notifications

[mmeest]

close #1525

Summary

• Update email texts for winner, invoice reminder, and ban notifications in Estonian and English to match the official samples.
• Introduce a separate final ban email sent from the third overdue invoice onwards.

Details
Adjusted ban_mailer logic to send a different email for the 3rd and subsequent unpaid invoices.
Added a new final_ban_mail mailer action and HTML view.
Updated I18n YAML files for result_mailer, invoice_mailer, and ban_mailer (ET/EN) for wording, grammar, and clickable links.
Updated mailer previews and tests to cover the new emails and the changed content.

Test plan
Ran bin/rails test (excluding the known wkhtmltopdf-related failure).
Verified all updated emails via Rails mailer previews:
Winner emails (ET/EN)
Invoice reminder emails (ET/EN)
Ban emails for 1st/2nd strike (ET/EN)
Final ban emails for 3rd+ strike (ET/EN)
Manually checked that all links in the emails are clickable and point to the correct URLs.

[mmeest]

Updated Gemfile constraints:
nokogiri '>= 1.19.1'
rack '~> 3.2.5'

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud