refactor: 统一 AGC 输入区为单框结构

- form 作为完整背景框承载输入区和底部控制行

- 输入编辑器在框内透明显示

- 模型选择、@ 和发送按钮位于同一框内底部行

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

@@ -9040,16 +9040,19 @@ iframe.preview-frame {
   display: grid;
   grid-template-columns: minmax(0, 1fr);
   grid-template-rows: auto auto;
-  gap: 8px;
+  gap: 0;
   align-items: stretch;
   position: absolute;
   right: 0;
   bottom: 0;
   left: 0;
   z-index: 5;
-  padding: 0;
-  background: transparent;
-  pointer-events: none;
+  padding: 10px 12px 8px;
+  border: 1px solid var(--platform-surface-border);
+  border-radius: 14px;
+  background: var(--platform-input-fill);
+  box-shadow: 0 8px 24px rgb(31 41 55 / 8%);
+  pointer-events: auto;
 }
 
 .game-workbench-chat
@@ -9057,9 +9060,29 @@ iframe.preview-frame {
   .project-supervisor-composer.is-direct-codex
   .resource-reference-input {
   width: 100%;
-  min-height: 72px;
-  padding: 12px 15px 10px;
-  pointer-events: auto;
+  min-height: 64px;
+  padding: 0 0 6px;
+  border: 0;
+  border-radius: 0;
+  background: transparent;
+  box-shadow: none;
+}
+
+.game-workbench-chat
+  .project-supervisor-surface.is-direct-codex
+  .project-supervisor-composer.is-direct-codex
+  .resource-reference-input:focus-within {
+  border: 0;
+  outline: none;
+  box-shadow: none;
+}
+
+.game-workbench-chat
+  .project-supervisor-surface.is-direct-codex
+  .project-supervisor-composer.is-direct-codex
+  .resource-reference-input-placeholder {
+  top: 0;
+  left: 0;
 }
 
 .game-workbench-chat
@@ -9071,6 +9094,7 @@ iframe.preview-frame {
   justify-content: flex-end;
   gap: 8px;
   min-width: 0;
+  padding-top: 4px;
   pointer-events: auto;
 }