feat(ui): bundle Inter + design-system preview page (/app/design)

Slice 2: self-host Inter (variable) via @fontsource-variable/inter and add a
living design-system reference at /app/design showing tokens, the green ramp,
typography, and all primitives in both themes. Verified: frontend builds clean;
prod npm audit clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -9,6 +9,7 @@
     "preview": "vite preview --host"
   },
   "dependencies": {
+    "@fontsource-variable/inter": "^5.2.8",
     "@radix-ui/react-avatar": "^1.2.1",
     "@radix-ui/react-checkbox": "^1.3.6",
     "@radix-ui/react-dialog": "^1.1.18",