feat(web): scaffold Next.js 16 dashboard with design system and auth client

Build out apps/web with Tailwind CSS v4, BetterAuth client, and the Mosaic
design system. Populate @mosaic/design-tokens with actual color palette
(deep blue-grays, blue/purple/teal accents), font definitions (Outfit/Fira Code),
and spacing scale per AD-13.

- Tailwind v4 CSS-first config with @theme tokens in globals.css
- next/font for self-hosted Outfit + Fira Code (no Google CDN)
- App shell with sidebar nav, topbar, and content area
- Route groups: (dashboard) with AppShell, (auth) with centered card
- Placeholder pages: /chat, /login, /register
- BetterAuth React client (useSession, signIn, signUp, signOut)
- Gateway API fetch wrapper with credentials
- cn() utility with clsx + tailwind-merge
- Dark theme default, WCAG-friendly contrast

Refs #26

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/design-tokens/package.json

@@ -1,6 +1,7 @@
 {
   "name": "@mosaic/design-tokens",
   "version": "0.0.0",
+  "type": "module",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
   "exports": {