Fix 2FA QR code rendering

[flemzord]

What changed

• Strip only the XML declaration from the generated QR-code SVG.
• Add a controller regression test verifying that the enrollment page contains the SVG and its otpauth link.

Why

rqrcode 3.x serializes the XML declaration and SVG on one line. The previous greedy expression matched from the XML declaration through the end of that line, deleting the entire SVG and leaving the 2FA enrollment page without a QR code.

Impact

Users can once again scan the QR code when enrolling in TOTP two-factor authentication.

Validation

• bundle exec rspec spec/controllers/settings_controller_spec.rb
• bundle exec standardrb app/controllers/settings_controller.rb spec/controllers/settings_controller_spec.rb
• Docker image build using the project Dockerfile