feat(i18n): add Polish localization

[Dragonk]

Summary

Adds complete Polish (pl) localization support to MailFlow.

The change includes the Polish UI translation, language selection and persistence, Polish-aware AI language instructions, and Polish pluralization support.

Changes

• add frontend/src/locales/pl.json
• register Polish in the i18next configuration
• add Polski to the language selector
• persist pl using the existing language preference flow
• add Polish to the backend AI language mapping
• add/update AI language mapping tests
• add Polish-specific plural forms where required
• extend the i18n tests to support locale-specific plural keys
• add a real i18next plural-resolution test for Polish
• update the README language list

Polish pluralization

Polish plural resolution is tested through i18next for representative values:

• 1
• 2
• 5
• 21
• 22
• 25
• 101
• 102
• 111

This verifies the one, few, and many forms used by Polish.

Validation

The branch was validated with:

• i18n tests: 1320/1320 passing
• frontend unit tests: 1574/1574 passing
• frontend lint: passing
• frontend production build: passing
• backend tests: 1101/1101 passing
• backend lint: passing
• git diff --check: passing

Notes

The translation was reviewed specifically for Polish grammar, terminology, plural forms, interpolation placeholders, and common email-client terminology.

Technical terms and established product/protocol names such as IMAP, SMTP, OAuth, OIDC, SSO, MFA, CardDAV, Gmail, Outlook, iCloud, Todoist, and Reply-To are intentionally preserved where appropriate.

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Contributor License Agreement

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• I have read and agree to the Contributor License Agreement.
• My contribution is my own original work (or I have identified any third-party material and confirmed it is compatible with the CLA).
• I have the right to submit this contribution under the terms of the CLA.

[Dragonk]

Hi @maathimself - just a small ping in case this PR got buried.

The Polish localization is ready for review and the CLA is complete. The branch has been kept up to date with main, and the frontend/backend validation passes locally.

GitHub Actions is currently waiting for maintainer approval because this is my first contribution to MailFlow, so the required Backend/Frontend checks cannot start until the workflow is approved.

When convenient, could you please approve the workflow run? Thanks!

[maathimself]

lgtm