refactor: remove legacy app-config.json system

Replace dual-configuration architecture (Remnawave API + local file fallback)
with Remnawave-only approach. When config is unavailable, show explicit
"not configured" message instead of silent file fallback.

- Delete app-config.json and admin_apps.py CRUD module (~1260 lines)
- Remove sync loaders, legacy step format handlers, device_mapping dicts
- Remove miniapp /app-config.json endpoint and filesystem search
- Remove backup service app-config.json snapshot/restore
- Remove APP_CONFIG_PATH setting, env var, docker volume mount
- Remove hardcoded 6-device keyboard fallback
- Remove legacy step-based keyboard rendering (installationStep etc.)
- Add "config not configured" message when Remnawave config is missing
- Update admin UI: "clear config" disables guide mode instead of reverting

.env.example

@@ -803,8 +803,6 @@ PRICE_ROUNDING_ENABLED=true
 TZ=Europe/Moscow # или UTC, America/New_York и т.д.
 
 # ===== ДОПОЛНИТЕЛЬНЫЕ НАСТРОЙКИ =====
-# Конфигурация приложений для гайда подключения
-APP_CONFIG_PATH=app-config.json
 ENABLE_DEEP_LINKS=true
 APP_CONFIG_CACHE_TTL=3600