Add ICU plural/select support, new locale catalogs, and en-XA pseudo-locale

[aishabellova]

Summary

Add ICU plural/select support, new locale catalogs, and en-XA pseudo-locale

What was solved

#495 — [High] Add new locale catalogs (pt-BR, fr, zh-CN) with ICU plurals, placeholder parity checks and a pseudo-locale for testing

Add ICU-style plural/select support to the i18n system via Intl.PluralRules, introduce pt-BR/fr/zh-CN catalogs plus a generated en-XA pseudo-locale, extend the parity checker to validate placeholders/plural categories/lengths, and wire the pseudo-locale into dev via ?lang=en-XA and the Storybook toolbar.

Addressed:

• Changed: src/lib/i18n/index.ts, src/lib/i18n/messages/en.ts, scripts/check-i18n-parity.mjs
• Catalog entries must support the ICU subset {count, plural, one {…} other {…}} (and select) resolved via Intl.PluralRules(locale).
• The interpolate function must validate arguments and never throw — falling back to the key on error.
• Add three fully translated locale catalogs: pt-BR, fr, zh-CN (RTL explicitly out of scope).

Changes

• src/lib/i18n/index.ts (modify)
• src/lib/i18n/messages/en.ts (modify)
• scripts/check-i18n-parity.mjs (modify)

Approach

1. Extend src/lib/i18n/index.ts with an ICU-subset parser/resolver (plural/select via Intl.PluralRules, {n, number} formatting, escaped braces) and a non-throwing interpolate that falls back to the key on error.
2. Add pt-BR, fr, and zh-CN catalogs under src/lib/i18n/messages/ typed against the en catalog, plus a generated en-XA pseudo-locale (accented chars, ~40% expansion, brackets) exposed in dev via ?lang=en-XA.
3. Extend scripts/check-i18n-parity.mjs to validate identical key sets, matching placeholder names, valid plural categories per locale, no empty strings, and max length ratio warnings without breaking existing en/es checks.
4. Replace the manual plural ternary in src/app/contributors/ContributorsPageClient.tsx with the new ICU plural mechanism.
5. Wire the en-XA pseudo-locale into the Storybook toolbar in .storybook/preview.tsx.
6. Add parser tests (≥30 cases), parity-script fixture tests, and contributor docs on adding/reviewing a locale.

Issues

Closes #495

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