fix: soften the transition above the chat composer

Add a theme-aware 16px fade above the composer to soften the clipped message edge without changing layout or pointer interactions. Document the treatment in the UI design system.

Release note: 聊天输入区顶部增加淡渐变，让消息与输入框之间的衔接更自然，并适配浅色和深色主题。

src/renderer/src/styles.css

@@ -7785,6 +7785,7 @@ button > svg {
 }
 
 .composer-wrap {
+  position: relative;
   width: 100%;
   max-width: 100%;
   min-width: 0;
@@ -7795,6 +7796,17 @@ button > svg {
   background: var(--surface-raised);
 }
 
+.composer-wrap::before {
+  position: absolute;
+  right: 0;
+  bottom: 100%;
+  left: 0;
+  height: var(--space-4);
+  background: linear-gradient(to bottom, transparent, var(--surface-raised));
+  content: "";
+  pointer-events: none;
+}
+
 .composer {
   width: 100%;
   max-width: 100%;