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/breakpoints.css

@@ -0,0 +1,11 @@
+/**
+ * 响应式断点 token
+ * 原始 Dart: lib/ui/core/responsive.dart
+ */
+:root {
+  --bp-mobile-max: 599.98px;
+  --bp-tablet-min: 600px;
+  --bp-tablet-max: 1023.98px;
+  --bp-desktop-min: 1024px;
+  --shell-max-width: 500px;
+}