fix: improve AI chat attachment close button visibility on dark theme

Use subtle translucent white background with soft text opacity instead
of opaque dark background that was invisible on dark theme.

Author: abose

src/styles/Extn-AIChatPanel.less

@@ -1862,15 +1862,21 @@
                 width: 16px;
                 height: 16px;
                 border-radius: 50%;
-                background: rgba(0, 0, 0, 0.7);
-                border: 1px solid rgba(255, 255, 255, 0.2);
-                color: @project-panel-text-2;
+                background: rgba(255, 255, 255, 0.12);
+                border: 1px solid rgba(255, 255, 255, 0.18);
+                color: rgba(255, 255, 255, 0.7);
                 font-size: 11px;
                 padding: 0;
                 display: none;
                 align-items: center;
                 justify-content: center;
                 cursor: pointer;
+                transition: background 150ms, color 150ms;
+
+                &:hover {
+                    background: rgba(255, 255, 255, 0.22);
+                    color: #fff;
+                }
             }
 
             &:hover .ai-image-remove {
@@ -1904,15 +1910,21 @@
                 width: 16px;
                 height: 16px;
                 border-radius: 50%;
-                background: rgba(0, 0, 0, 0.7);
-                border: 1px solid rgba(255, 255, 255, 0.2);
-                color: @project-panel-text-2;
+                background: rgba(255, 255, 255, 0.12);
+                border: 1px solid rgba(255, 255, 255, 0.18);
+                color: rgba(255, 255, 255, 0.7);
                 font-size: 11px;
                 padding: 0;
                 display: none;
                 align-items: center;
                 justify-content: center;
                 cursor: pointer;
+                transition: background 150ms, color 150ms;
+
+                &:hover {
+                    background: rgba(255, 255, 255, 0.22);
+                    color: #fff;
+                }
             }
 
             &:hover .ai-file-remove {