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/icon-sizes.css

@@ -0,0 +1,17 @@
+/**
+ * 图标尺寸 token
+ * 原始 Dart: lib/design/icon_sizes.dart
+ */
+@theme {
+  --icon-size-sm: 16px;
+  --icon-size-md: 18px;
+  --icon-size-lg: 20px;
+  --icon-size-xl: 24px;
+  --icon-size-xxl: 28px;
+  --icon-size-xxxl: 32px;
+  --icon-size-34: 34px;
+  --icon-size-40: 40px;
+  --icon-size-43: 43px;
+  --icon-size-56: 56px;
+  --icon-size-86: 86px;
+}