fix: add CSS module type declarations for dts generation

vite-plugin-dts runs tsc to generate declarations and needs explicit
type definitions for .module.css, .css, and .svg imports. Without
these, the DTS step fails with TS2307 for every CSS module import.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/design-system/types/css-modules.d.ts

@@ -0,0 +1,14 @@
+declare module '*.module.css' {
+  const classes: { readonly [key: string]: string }
+  export default classes
+}
+
+declare module '*.css' {
+  const css: string
+  export default css
+}
+
+declare module '*.svg' {
+  const url: string
+  export default url
+}