fix(ci): update gitlab ci to official vercel pipeline template

.gitlab-ci.yml

 # GitLab CI/CD Pipeline for deploying Next.js to Vercel using Vercel CLI
-# Reference: https://vercel.com/kb/guide/how-can-i-use-gitlab-pipelines-with-vercel
+# Standard template matching official Vercel documentation:
+# https://vercel.com/kb/guide/how-can-i-use-gitlab-pipelines-with-vercel
 
-image: node:20-alpine
+image: node:20
 
 stages:
   - deploy
 
-variables:
-  PNPM_HOME: "/root/.local/share/pnpm"
-  PATH: "$PNPM_HOME:$PATH"
-
-before_script:
-  - npm install -g pnpm vercel
-  - pnpm config set store-dir .pnpm-store
-
-# Cache pnpm dependencies and Next.js build cache to speed up pipelines
-cache:
-  key:
-    files:
-      - pnpm-lock.yaml
-  paths:
-    - .pnpm-store
-    - .next/cache
-
-# 1. Preview Deployment (Triggered on feature branches and Merge Requests)
+# 1. Preview Deployment (Tất cả các nhánh tính năng trừ staging, main)
 deploy_preview:
   stage: deploy
-  rules:
-    - if: '$CI_COMMIT_BRANCH != "main" && $CI_COMMIT_BRANCH != "master"'
+  except:
+    - staging
+    - main
+  before_script:
+    - npm install --global pnpm vercel
   script:
     - vercel pull --yes --environment=preview --token=$VERCEL_TOKEN
     - vercel build --token=$VERCEL_TOKEN
@@ -36,11 +23,14 @@ deploy_preview:
     VERCEL_ORG_ID: $VERCEL_ORG_ID
     VERCEL_PROJECT_ID: $VERCEL_PROJECT_ID
 
-# 2. Production Deployment (Triggered only when merging into main/master)
+# 2. Production Deployment (Kích hoạt khi push lên nhánh staging, main)
 deploy_production:
   stage: deploy
-  rules:
-    - if: '$CI_COMMIT_BRANCH == "main" || $CI_COMMIT_BRANCH == "master"'
+  only:
+    - staging
+    - main
+  before_script:
+    - npm install --global pnpm vercel
   script:
     - vercel pull --yes --environment=production --token=$VERCEL_TOKEN
     - vercel build --prod --token=$VERCEL_TOKEN