fix: correct BUILD_BRANCH from 'main' to 'master'

The repository uses 'master' branch, not 'main'. This mismatch
was causing /cc and /webrtc routes to not work on Cloudflare Pages.

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

Jenkinsfile

@@ -11,7 +11,7 @@ pipeline {
         PROJECT_NAME = 'tg-live-game-web'
 
         // 分支配置
-        BUILD_BRANCH = 'main'
+        BUILD_BRANCH = 'master'
 
         // Node 配置 22.21.1 也是可用的
         NODE_VERSION = 'v20.19.3'