[SandObserver]

Summary

• Store each counted message once per CLDR plural category (key_one, key_other, ...) instead of a singular key plus a ...Plural twin, and rename their {n} placeholder to {count}.
• Select the form in t() with Intl.PluralRules against the active locale, resolving active category, active other, English category, English other, then the key.
• Replace the five count === 1 call sites with t(key, { count }) and delete refusedNoticeKey, whose only job was picking between the two keys.
• Give badge-logic.js the same category resolution over its injected-translator English fallback map, keeping the module import-free.
• Extend the locale registry with an English name and a source/machine status, and export SOURCE_LANG and isSupported.
• Resolve direction for an unregistered tag from Intl.Locale text info, keeping the language-subtag set as the fallback for runtimes that carry none.
• Add two URL-only development locales that are absent from the registry: en-XA accents, pads and brackets every message; cimode loads no catalog so each string renders as its key.
• Add gates for plural-category completeness per locale, placeholder parity against English, registry-to-file agreement, per-widget catalog locale and key coverage, and pseudolocalisation leaving placeholders and markup intact.
• Teach the reachability test to compare base names, so a plural category the source never writes out is still accounted for.
• Rewrite docs/i18n.md around the registry table, the per-key fallback policy, the counted-message workflow, a glossary, the development locales, and the validation command.

Testing
All green: 1080 frontend tests, the backend suite at 94.35% lines, lint, format, both typechecks, the cache-busting check, and the Docker build. Added 15 tests covering plural categories across the six locales, decimal and zero counts, catalog and placeholder parity, the registry, and the pseudolocale transform.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.