Add mobile app starter template

[solethus]

Summary

• Adds ts/mobile-starter — a full-stack monorepo starter with an Encore.ts backend, Vite/React web app, and Expo/React Native mobile app
• Uses WorkOS for authentication: email/password, OAuth (Google/Microsoft), email verification, password reset
• Implements organization management with role-based access control (Admin/Member) and member invitations
• Installable via encore app create my-app --example=ts/mobile-starter

What's included

• backend/auth/ — Encore.ts auth service with WorkOS integration (14 files)
• web/ — Vite + React 19 + TanStack Router with auth provider, permission guards, login/signup/OAuth forms, dashboard/profile/members pages
• native/ — Expo 54 + React Native with SecureStore-based auth, OAuth via expo-web-browser, tab navigation (Dashboard/Members/Profile)
• Monorepo config: Bun workspaces, Turborepo, Biome, Taskfile

Test plan

• encore run starts backend, endpoints visible at localhost:9400
• cd web && bun install && bun run dev starts web app at localhost:3001
• cd native && bun install && bun run dev starts Expo dev server
• Sign up / sign in flows work on web and native
• OAuth login redirects correctly
• Admin users can invite members; Member users see read-only member list
• Token refresh works silently