Refactor chat component styles for responsiveness and consistency

- Updated padding and margin properties across various chat components to utilize responsive design tokens, ensuring better adaptability to different screen sizes.
- Replaced fixed sizes with responsive variables for buttons, avatars, and bubbles to enhance UI flexibility.
- Adjusted font sizes to use responsive tokens, improving text readability on various devices.
- Standardized border-radius and other dimensions using CSS variables for a cohesive design language.
- Enhanced dialog and screen components with safe area insets for better compatibility with modern devices.

src/tokens/dimensions.css

@@ -51,7 +51,7 @@
   --auth-legal-checkbox-inner-size: 10px;
 
   /* 悬浮返回按钮（40×40 圆形） */
-  --back-button-size: 40px;
+  --back-button-size: var(--responsive-icon-button-size, 40px);
 
   /* 底部弹层圆角（28px = Dart AppRadius.radius28） */
   --auth-bottom-sheet-radius: 28px;

src/tokens/responsive.css

@@ -40,6 +40,13 @@
   --responsive-control-height: clamp(44px, 8.889vw, 48px);
   --responsive-icon-button-size: clamp(38px, 8.148vw, 44px);
 
+  /* Chat 专用响应式尺寸 */
+  --chat-inline-padding: clamp(14px, 3.704vw, 20px);
+  --chat-bubble-max-width: min(75%, 280px);
+  --chat-media-size: min(64vw, 240px);
+  --chat-avatar-size: clamp(38px, 7.963vw, 43px);
+  --chat-send-button-size: clamp(40px, 8.148vw, 44px);
+
   /* Sidebar 专用响应式尺寸 */
   --sidebar-user-padding-y: clamp(18px, 3.704vw, 20px);
   --sidebar-card-gap-y: clamp(6px, 1.481vw, 8px);