ci: 构建流程切换为官方Gradle Action

将原有自定义Gradle缓存步骤替换为gradle/actions/setup-gradle官方Action，简化配置并利用内置智能缓存，提升CI流程维护性与稳定性。

.github/workflows/build.yml

@@ -72,17 +72,11 @@ jobs:
       - name: Setup Android SDK
         uses: android-actions/setup-android@v3
 
-      # 缓存 Gradle 依赖和构建缓存
-      - name: Cache Gradle
-        uses: actions/cache@v4
+      # 使用官方 Gradle Action（内置智能缓存）
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
         with:
-          path: |
-            ~/.gradle/caches
-            ~/.gradle/wrapper
-            ~/.gradle/build-cache
-          key: gradle-${{ runner.os }}-${{ hashFiles('android/**/*.gradle*', 'android/**/gradle-wrapper.properties') }}
-          restore-keys: |
-            gradle-${{ runner.os }}-
+          cache-read-only: false
 
       - name: Build Frontend
         run: |