feat(MNG): FCM 채널명 동기화 및 config 일원화 (7채널)

- push_urgent → push_vendor_register (거래처등록)
- push_payment → push_approval_request (결재요청)
- push_income 신규 추가 (입금)
- config/fcm.php 전체 7개 채널 등록
- FcmController validation 및 Blade select 옵션 반영

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config/fcm.php

@@ -38,8 +38,13 @@
     |
     */
     'channels' => [
-        'default' => 'push_default',
-        'urgent' => 'push_urgent',
+        'default'          => 'push_default',
+        'vendor_register'  => 'push_vendor_register',
+        'approval_request' => 'push_approval_request',
+        'income'           => 'push_income',
+        'sales_order'      => 'push_sales_order',
+        'purchase_order'   => 'push_purchase_order',
+        'contract'         => 'push_contract',
     ],
 
     /*