feat(web): 设置中心缓存管理与文件预览资产工具

- 新增 documentPreviewAssets 工具，统一从 URL/Blob/File 推断预览类型（image/pdf/file/unsupported）
- SettingsView/SettingsView.js/settingsModelHelper 新增系统缓存管理区块，调用 /settings/cache/clear 并展示清理结果；useSettings/services 适配
- WorkbenchAiFilePreviewDialog/useWorkbenchAiFilePreview 接入预览资产工具，workbenchAiComposerModel 调整文件处理
- ReceiptFolder/LogDetailView/DigitalEmployeeWorkRecords/travelReimbursementAttachmentModel 配套适配
- 新增 settings-cache-management-section 测试，更新 settings-llm/rendering/receipt-folder-view/composer-components/attachment-association 测试

web/src/components/business/workbench-ai/WorkbenchAiFilePreviewDialog.vue

@@ -36,6 +36,10 @@
               class="workbench-ai-file-preview-frame"
               title="附件 PDF 预览"
             ></iframe>
+            <div v-else-if="preview.sourceKind === 'loading'" class="workbench-ai-file-preview-state">
+              <i class="mdi mdi-loading"></i>
+              <span>正在加载统一预览。</span>
+            </div>
             <div v-else class="workbench-ai-file-preview-state">
               <i class="mdi mdi-file-eye-outline"></i>
               <span>当前附件暂不支持直接预览。</span>