refactor(design-tokens): move @theme declarations to individual token files

Consolidate the design system registration by removing the centralized
`@theme inline` block and all design token imports from globals.css.
Each token file now declares its own `@theme` rules directly, allowing
Tailwind 4 to auto-collect them and generate utility classes (e.g.
`bg-accent`, `p-md`, `rounded-lg`) without redundant re-mapping.

This simplifies globals.css, removes a layer of indirection, and keeps
the design token definitions co-located with their `@theme` exposure.

src/tokens/dimensions.css

@@ -0,0 +1,35 @@
+/**
+ * 固定尺寸 token
+ * 原始 Dart: lib/design/app_dimensions.dart
+ *
+ * 注：这些是不应通过 Tailwind 缩放变化的硬尺寸（如按钮高度、对话框宽度）
+ */
+:root {
+  /* 按钮 */
+  --button-height: 48px;
+  --button-height-lg: 56px;
+
+  /* Splash Facebook 登录按钮 */
+  --splash-facebook-button-height: 56px;
+  --splash-facebook-button-radius: 28px;
+
+  /* PWA 按钮 */
+  --pwa-button-height: 44px;
+
+  /* 对话框 */
+  --dialog-max-width: 360px;
+  --pwa-install-dialog-max-width: 360px;
+  --pwa-install-container-size: 64px;
+
+  /* 列表项 */
+  --item-height: 56px;
+
+  /* 消息气泡 */
+  --bubble-min-width: 60px;
+  --bubble-max-width: 280px;
+  --bubble-min-height: 36px;
+
+  /* 聊天媒体 */
+  --chat-media-max-width: 240px;
+  --chat-media-max-height: 240px;
+}