Refactor bot handlers and keyboards; implement admin features

- Removed unused prehandling, referrals, support, and states files.
- Updated admin and client handlers to improve structure and functionality.
- Introduced admin filters and routers for better access control.
- Added new billing and deposit functionalities for client interactions.
- Enhanced keyboard layouts for admin and client menus.
- Implemented promotion handling for admin users.
- Updated configuration to include admin settings.
- Improved utility functions for quote fetching and menu text building.
- Added common handlers for undefined commands and message deletions.

bot/keyboards/common.py

@@ -0,0 +1,12 @@
+from aiogram.types import InlineKeyboardButton, InlineKeyboardMarkup
+from aiogram.utils.keyboard import InlineKeyboardBuilder
+
+from config import settings
+from misc.utils import convert_duration_to_int
+from schemas.subscriptions import SubscriptionDuration
+
+_return_to_menu_btn: InlineKeyboardButton = InlineKeyboardButton(
+    text="⬅️ Назад", callback_data="menu:main"
+)
+
+return_to_menu: InlineKeyboardMarkup = InlineKeyboardBuilder([[_return_to_menu_btn]]).as_markup()