feat(ui): show plain MIME alternative

[bhagatsaurabh]

Fixes #13466
Fixes #8819

Adds a message action to switch between HTML and plain-text MIME versions when both are available. This provides a readable fallback for emails with broken HTML layouts.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[ChristophWurst]

Thanks for the contribution

[ChristophWurst]

This contribution appears to be assisted by AI. Please see AGENTS.md and CONTRIBUTING.md. Restore the PR template and make the model usage transparent. Thanks

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