Refactor file handling in LibraryMergeService to use move instead of copy for audio, lyrics, and cover files. Update MergeTab.vue to include a warning about files being moved to the main library.

frontend/src/components/MergeTab.vue

@@ -36,6 +36,13 @@
               </el-input>
             </el-form-item>
 
+            <div class="strategy-tip merge-tip">
+              <el-icon class="tip-icon"><Warning /></el-icon>
+              <div class="tip-content">
+                <strong>重要提示</strong>：文件会<strong>移动</strong>到主库（源目录 staging 中的文件会被移走，不保留）；歌词和封面一并移动。
+              </div>
+            </div>
+
             <el-form-item label="合并策略">
               <el-checkbox v-model="form.smartUpgrade">启用智能升级</el-checkbox>
               <el-checkbox v-model="form.keepBackup">保留旧版本备份</el-checkbox>