style(chat): align header and input styles with Dart implementation

Update chat-header and chat-input-text-field CSS to match the
Dart counterparts (chat_header.dart, chat_input_text_field.dart):
- Header: remove outer padding/background, use transparent base
- Guest banner: switch to --spacing-sm/md tokens
- Header row: add margin-left and symmetric padding
- Menu button: use 32px radius and #0d0b14 @ 0.7 background
- Input field: add white background, 32px radius, accent caret
  color, and flex centering for vertical text alignment

src/app/chat/components/chat-header.module.css

@@ -1,21 +1,22 @@
-/* ChatHeader 顶部栏样式 */
+/* ChatHeader 顶部栏样式（与 Dart chat_header.dart 对齐） */
 
+/* 外层：等价于 Dart `Column(mainAxisSize: MainAxisSize.min)` — 无 padding/背景 */
 .header {
   flex: 0 0 auto;
   display: flex;
   align-items: center;
-  justify-content: space-between;
-  padding: var(--spacing-4, 16px) var(--spacing-5, 20px);
-  background: var(--color-header-background, rgba(0, 0, 0, 0.3));
+  padding: 0;
+  background: transparent;
 }
 
+/* 游客模式横幅（Dart: Container(color: accent, padding: vertical 8 / horizontal 12)） */
 .guestBanner {
   width: 100%;
   display: flex;
   align-items: center;
   justify-content: center;
-  gap: var(--spacing-2, 8px);
-  padding: var(--spacing-2, 8px) var(--spacing-4, 16px);
+  gap: var(--spacing-sm, 8px);
+  padding: var(--spacing-sm, 8px) var(--spacing-md, 12px);
   background: var(--color-accent, #f84d96);
   color: #fff;
   font-size: var(--font-size-sm, 12px);
@@ -30,21 +31,24 @@
   height: var(--icon-size-sm, 16px);
 }
 
+/* 登录模式行容器（Dart: Container(margin-left 12, padding: vertical 8 / horizontal 12)） */
 .headerRow {
   width: 100%;
   display: flex;
   align-items: center;
   justify-content: space-between;
-  padding: 0 var(--spacing-4, 16px);
+  margin-left: var(--spacing-md, 12px);
+  padding: var(--spacing-sm, 8px) var(--spacing-md, 12px);
 }
 
+/* 菜单按钮（Dart: Container(padding 8, bg #0d0b14 @ 70%, radius 32)） */
 .menuButton {
   display: flex;
   align-items: center;
   justify-content: center;
-  padding: var(--spacing-2, 8px);
-  background: var(--color-dark-background, #111)cc;
-  border-radius: 9999px;
+  padding: var(--spacing-sm, 8px);
+  background: rgba(13, 11, 20, 0.7); /* AppColors.darkBackground #0d0b14 @ alpha 0.7 */
+  border-radius: 32px; /* AppSpacing.xxxl */
   border: 0;
   cursor: pointer;
   color: var(--color-text-primary, #fff);