feat(settings): per-user language preference and panel improvements

Store UI language per user (users.language column, migration 26).
Language selector moves from Panel tab to Profile tab — each account
picks its own language, saved to DB and applied via i18n.changeLanguage()
on save so the interface switches immediately.

Panel tab: panel_name read-only (falls back to env PANEL_NAME),
log_retention_days field exposed in UI, language selector removed.
Panel API accepts panel_name in PUT so it can be overridden in DB
without touching the env var.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/hooks/use-settings.ts

@@ -41,6 +41,7 @@ interface ProfileResponse {
   is_admin: boolean;
   is_active: boolean;
   ssh_public_key?: string;
+  language: string;
   created_at: string;
   last_login?: string;
 }

frontend/src/locales/en.json

@@ -681,6 +681,8 @@
     "session_timeout": "Session Timeout",
     "seconds": "seconds",
     "minutes": "minutes",
+    "days": "days",
+    "log_retention_days": "Log Retention",
     "security_saved": "Security settings saved",
     "version": "Version",
     "check_updates": "Check for Updates",

frontend/src/locales/ru.json

@@ -681,6 +681,8 @@
     "session_timeout": "Таймаут сессии",
     "seconds": "секунд",
     "minutes": "минут",
+    "days": "дней",
+    "log_retention_days": "Хранение логов",
     "security_saved": "Настройки безопасности сохранены",
     "version": "Версия",
     "check_updates": "Проверить обновления",