i18n: translate Electron main-process menus

[Davidnet]

Part of the frontend internationalization effort (see Epic). Depends on language selector + persistence.

The Electron main process is separate from React and has ~40 hardcoded label:/title: strings (tray menu, app menu, tooltips) in src/frontend/src/electron/electron-main.js.

Tasks

• Load standalone i18next in the main process, sharing the same locale JSON files
• Read the persisted locale on startup
• Translate tray menu, app menu, and tooltips
• Rebuild the menu on a language-change IPC event (renderer → main)