Enlarge collapse toggle hit area for easier clicking

Adds padding with negative margin to both widget and agent header
collapse toggles, making the clickable zone ~34px without changing
the visual arrow size.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/style.css

@@ -681,11 +681,16 @@ body {
 .agent-header .collapse-toggle {
     width: 8px;
     height: 8px;
+    padding: 10px;
+    margin: -10px;
+    box-sizing: content-box;
     display: flex;
     align-items: center;
     justify-content: center;
     cursor: pointer;
     flex-shrink: 0;
+    position: relative;
+    z-index: 1;
 }
 
 .agent-header .collapse-toggle::before {
@@ -4198,12 +4203,18 @@ pre code [class*="token"] {
 .widget-collapse-toggle {
     width: 14px;
     height: 14px;
+    padding: 10px;
+    margin: -10px;
+    box-sizing: content-box;
     display: flex;
     align-items: center;
     justify-content: center;
     flex-shrink: 0;
     opacity: 0.6;
+    cursor: pointer;
     transition: opacity 0.2s;
+    position: relative;
+    z-index: 1;
 }
 
 .widget-collapse-toggle:hover {