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/border-widths.css

@@ -0,0 +1,12 @@
+/**
+ * 边框宽度 token
+ * 原始 Dart: lib/design/border_widths.dart
+ */
+@theme {
+  --border-thin: 0.5px;
+  --border-light: 1px;
+  --border-normal: 1.5px;
+  --border-medium: 2px;
+  --border-bold: 3px;
+  --border-heavy: 4px;
+}