feat: add Telegram customers management system with admin panel

Implement comprehensive Telegram customers storage and management functionality:

Backend:
- Add database migration for telecart_customers table with indexes
- Create TelegramCustomer model with CRUD operations
- Implement TelegramCustomerService for business logic
- Add TelegramCustomerHandler for API endpoint (saveOrUpdate)
- Add TelegramCustomersHandler for admin API (getCustomers with pagination, filtering, sorting)
- Add SendMessageHandler for sending messages to customers via Telegram
- Create custom exceptions: TelegramCustomerNotFoundException, TelegramCustomerWriteNotAllowedException
- Refactor TelegramInitDataDecoder to separate decoding logic
- Add TelegramHeader enum for header constants
- Update SignatureValidator to use TelegramInitDataDecoder
- Register new routes in bastion/routes.php and src/routes.php

Frontend (Admin):
- Add CustomersView.vue component with PrimeVue DataTable
- Implement advanced filtering (text, date, boolean filters)
- Add column visibility toggle functionality
- Add global search with debounce
- Implement message sending dialog with validation
- Add Russian locale for PrimeVue components
- Add navigation link in App.vue
- Register route in router

Frontend (SPA):
- Add saveTelegramCustomer utility function
- Integrate automatic customer data saving on app initialization
- Extract user data from Telegram.WebApp.initDataUnsafe

The system automatically saves/updates customer data when users access the Telegram Mini App,
and provides admin interface for viewing, filtering, and messaging customers.

BREAKING CHANGE: None

frontend/admin/src/assets/main.css

@@ -66,3 +66,11 @@ ul.formkit-options label {
 ul.formkit-options input[type="radio"] {
   position: absolute;
 }
+
+input.p-checkbox-input {
+  position: absolute;
+}
+
+label {
+  margin-bottom: 0;
+}