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/handlers/system/__init__.py

@@ -0,0 +1,4 @@
+from .common import router as common_router
+from .prehandling import router as prehandling_router
+
+routers = [common_router, prehandling_router]