feat(编辑能力): 支持签名移除清空及撤销重做

assets/webui/index.html

@@ -876,7 +876,7 @@
 				<section id="signaturePanel" class="signature-panel" hidden>
 					<div class="panel-row">
 						<div id="signaturePanelTitle" class="panel-title compact">签名</div>
-						<span id="signatureSummary" class="count-pill">0</span>
+						<div class="panel-actions"><span id="signatureSummary" class="count-pill">0</span><button id="signatureClear" class="small-button" type="button" hidden>清空</button></div>
 					</div>
 					<div id="signatureList" class="signature-list"></div>
 				</section>