feat(渲染逻辑): 完善图片另存与缺字体处理

assets/webui/index.html

@@ -140,6 +140,7 @@
 					<option value="contain">适应</option>
 					<option value="cover">填充</option>
 				</select>
+				<button id="saveImageButton" class="button" type="button" disabled>另存</button>
 				<button id="cropImageButton" class="button" type="button" aria-label="裁剪图片" aria-pressed="false" disabled>裁剪</button>
 				<button id="resetCropButton" class="button" type="button" aria-label="还原图片" disabled>还原</button>
 				<span>图片</span>