Tiny reactive i18n module (locale ref, t() with {param} interpolation and
ru/uk plural forms, flat per-area dictionaries, en fallback) instead of a
vue-i18n dependency. App.vue and SecretDetailPanel.vue migrate every
hardcoded string to message slugs; /me's locale_effective (settings
override -> auth account -> en) seeds the language on load, the profile
modal gains a language GnSelect (Auto/English/Українська/Русский) that
PATCHes /me and re-applies the effective locale, and <html lang> follows.

Co-Authored-By: Claude Code <noreply@anthropic.com>