Add five-language website settings and localized API validation

[MaxDac]

Summary

• Add an English-default language setting for English, Italian, European Portuguese, Spanish and French, available before and after authentication. Persist the allowlisted preference independently in a browser-local cookie.
• Translate application-owned website/client presentation through shared Gettext catalogs and Phoenix LiveView. Switch in place while preserving drafts, encrypted/decrypted chat content, encryption state, safety digits, scroll, navigation and reconnect behavior; never translate user content.
• Localize human REST validation arrays using quality-weighted/regional Accept-Language negotiation. Preserve API v1, machine error codes, statuses, field/envelope shapes and socket protocols. Include localized unknown HTML-route errors and updated documentation.

Validation

• mix precommit passed: 124 domain and 153 web ExUnit tests, 140 JavaScript tests, strict Credo, formatting, TypeScript, ESLint and Prettier.
• Chromium: all 5 end-to-end scenarios passed; both locale preservation scenarios passed again after the final bundle rebuild.
• Final locale-focused ExUnit: 13/13 passed. Independent HTTP probes confirmed correctly accented validation messages for all five languages and negotiation behavior; catalog/resource parity and encoding checks passed. Staged diff whitespace check passed.

Caveats

Validation used WSL Elixir 1.20.4 / OTP 27 and Node 24.21.0 because Windows Erlang crashed. An earlier run reproduced the existing, unmodified PushTest's seed-dependent 100 ms asynchronous receive timeout; the final full precommit run passed. Publishing cleanup only removed extra catalog EOF blank lines; translation content and verified behavior were unchanged. No deployment or protocol/account-settings synchronization changes.