[Feature]: Add Russian (ru) localization support

[AlexZander85]

Problem Statement

Summary

I'm contributing a Russian localization for ClawX UI. This PR adds complete ru translations for all 9 i18n namespaces.

Changes

• Added 'ru' to SUPPORTED_LANGUAGE_CODES
• Created Russian translation files (common, settings, dashboard, chat, channels, agents, skills, cron, setup)
• Updated i18n/index.ts with Russian resources
• Added 'Русский' to language selector

PR

#798

Context

Russian is a widely used language with many OpenClaw/ClawX users. This contribution enables them to use the app in their native language.

Questions

• Is there a preferred translation convention for technical terms?
• Should I add more regional locale variants (ru-BY, ru-KZ, etc.)?
• Are there any additional files that need translation updates?

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Labels: i18n, localization, enhancement, community

Proposed Solution

#798

Alternatives Considered

No response

Success Criteria

Language selector — "Русский" appears in Settings > Language dropdown
All UI strings translated — All 9 namespace JSON files (common, settings, dashboard, chat, channels, agents, skills, cron, setup) contain Russian translations
No English fallback — When user selects Russian, UI renders in Russian (no fallback to English for translated keys)
Tests pass — pnpm run typecheck and pnpm run lint pass without errors
Verification command:

grep -r "Русский" src/i18n/ # should return the language entry

Additional Context

No response

[AlexZander85]

#798