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/spacing.css

@@ -0,0 +1,19 @@
+/**
+ * 间距 token
+ * 原始 Dart: lib/design/spacing.dart
+ */
+@theme {
+  --spacing-xs: 4px;
+  --spacing-sm: 8px;
+  --spacing-md: 12px;
+  --spacing-lg: 16px;
+  --spacing-xl: 20px;
+  --spacing-xxl: 24px;
+  --spacing-xxxl: 32px;
+
+  /* 扩展间距（迁移自 Flutter widgets） */
+  --spacing-26: 26px;
+  --spacing-30: 30px;
+  --spacing-40: 40px;
+  --spacing-46: 46px;
+}