feat(i18n): add Russian locale

[Tzeenchist]

Summary

• add RU to the config enum and frontend language registry
• register Russian locale resources in the i18n config
• add complete Russian translations for the existing locale namespaces
• align Russian git status keys with the English source locale

Stacking

This PR is stacked on top of #3297.
Please review/merge #3297 first, then review this PR commit-by-commit or after rebasing.

Validation

• node scripts/check-locale-translation.mjs ru
• node scripts/check-unused-i18n-keys.mjs
• ./scripts/check-i18n.sh

QA / review notes

• i18n validation is green
• browser QA was attempted, but the local environment has tooling blockers unrelated to the locale diff: broken dev:qa script, missing cargo-watch in dev, and Chromium sandbox restrictions in the browse tool

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Note

Low Risk
Low risk: changes are primarily additive i18n resources and docs, with a small config/type expansion (UiLanguage::RU) that could affect language selection but not core logic.

Overview
Adds Russian as a selectable UI language end-to-end by extending the backend UiLanguage enum / generated shared UiLanguage type and registering RU→ru plus the Russian endonym in the frontend language registry.

Registers Russian translation resources in the i18n config and introduces a full ru locale pack across the existing namespaces (common, settings, projects, tasks, organization).

Improves contributor workflow by documenting the i18n process (docs/i18n.md), adding locale helper scripts (scripts/create-locale.mjs, scripts/check-locale-translation.mjs), and wiring them into package.json as locale:new and locale:check.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Null values for nonexistent keys in cleanup section

Low Severity

The Russian locale adds "parallelLabel": null and "parallelHelper": null inside settings.repos.scripts.cleanup, but the English source locale (and every other locale) does not have these keys in the cleanup section — they only exist in the setup section. These are spurious keys with null values that don't correspond to any source key. Additionally, null values in JSON translation files bypass the get_json_keys and flattenStrings validation helpers (which filter for string-typed values), so the existing i18n consistency checks silently skip them.