修复消息内容中单换行的显示问题，通过组合 white-space 策略消除额外空行

src/styles.css

@@ -762,7 +762,7 @@ body {
   display: inline-block;
   padding: 8px;
   border-radius: 14px;
-  white-space: pre-line;
+  white-space: normal;
   word-wrap: break-word;
   line-height: 1.5;
   font-size: 14.5px;
@@ -794,14 +794,23 @@ body {
 .bubble.assistant h4,
 .bubble.assistant h5,
 .bubble.assistant h6,
+.bubble.assistant li {
+  margin: 0;
+  white-space: pre-wrap;
+}
+
 .bubble.assistant ol,
 .bubble.assistant ul {
   margin: 0;
   padding-left: 1.5em;
 }
 
-.bubble.assistant li {
-  margin: 0;
+.bubble.assistant p {
+  margin-bottom: 4px;
+}
+
+.bubble.assistant p:last-child {
+  margin-bottom: 0;
 }
 
 .bubble.assistant hr {