i18n: claim German (de) translation

[DevCurke]

Hi, I'd like to claim the German (de) locale per the
Contributing Translations guide.

Scope

• Frontend: src/ui/src/locales/de/{common,chat,modals,settings,sidebar}.json
• Backend: src/locales/de/tray.json
• Registration: src/ui/src/i18n.ts and src/i18n/mod.rs

Style

• Informal "you" form ("Du")
• Technical terms (Chat, Token, MCP, Tool Use, Worktree, Agent, Skill, Slash command) stay in English where they're the de-facto term in German dev communities.

Will open a draft PR once the smoke-test scope is ready.

Thanks!