feat(i18n): add Russian locale for web UI and server messages

[michael-xiii]

Summary

• Add Russian (ru) locale for the React web UI (i18next) and Spring backend message bundles.
• Follow-up to feat(i18n): add Russian locale (web UI + MessageSource) #588 (approved by @FenjuFu, closed without merge). Rebased on current main.

Changes since #588

• Rebased on main (includes PR feat(search): surface compliance mappings in discovery #699 common.* keys + updated compliance.title).
• Russian relative time in notification page and dropdown via shared formatRelativeTime helper.
• Translated agent setup prompts and email labels (zh precedent).
• Language switcher: adds Русский only — no unrelated fallback changes.

Behaviour notes

• fallbackLng remains en in i18n config (unchanged).
• ~20 UI strings intentionally identical to English (CLI commands, package names, OAuth, role codes) — listed below.

Intentionally untranslated

• landing.quickStart.cli.command: npm i -g @astron-team/skillhub
• landing.quickStart.human.command: npx clawhub search <keyword>
• OAuth tab labels, OWNER/ADMIN/MEMBER role codes, brand strings (SkillHub CLI)

Out of scope

• Docusaurus / VitePress documentation locales
• README_ru.md

Test plan

• Language switcher → Русский; core UI in Russian
• Notification dropdown + page show Russian relative time
• pnpm --dir web exec vitest run src/i18n/
• make typecheck-web && make lint-web
• CI: web-tests + server-tests (pr-tests.yml)

Related

• Supersedes feat(i18n): add Russian locale (web UI + MessageSource) #588
• @FenjuFu — prior LGTM; happy to adjust if anything blocked merge on feat(i18n): add Russian locale (web UI + MessageSource) #588