fix: 调整 AGC 输入区滚动条显示阈值

- 输入编辑器默认高度提高到 96px，超过 140px 后滚动

- 加深并加宽滚动条拇指，避免溢出时不可见

- 同步调整消息列表底部空间

apps/ai-game-creator-shell/src/styles.css

@@ -9211,17 +9211,17 @@ iframe.preview-frame {
 .game-workbench-chat
   .project-supervisor-surface.is-direct-codex
   .project-supervisor-message-list {
-  bottom: 116px;
+  bottom: 156px;
 }
 
 .resource-reference-input-editor::-webkit-scrollbar {
-  width: 8px;
+  width: 10px;
 }
 
 .resource-reference-input-editor::-webkit-scrollbar-thumb {
   border: 2px solid transparent;
   border-radius: 999px;
-  background: #cbd5e1;
+  background: #94a3b8;
   background-clip: padding-box;
 }
 
@@ -9281,8 +9281,9 @@ iframe.preview-frame {
   .project-supervisor-surface.is-direct-codex
   .project-supervisor-composer
   .resource-reference-input-editor {
-  min-height: 64px;
-  max-height: 160px;
+  min-height: 96px;
+  max-height: 140px;
+  overflow-y: auto;
   padding: 0;
   font-size: 13px;
   line-height: 1.6;