Add GitHub OAuth with Better Auth, D1, and Octokit

[stylessh]

Summary

• Adds GitHub OAuth authentication using Better Auth with the tanstackStartCookies plugin for TanStack Start
• Sets up Cloudflare D1 database with Drizzle ORM (user, session, account, verification tables) and initial migration
• Adds Octokit integration to fetch GitHub API on behalf of authenticated users (stored access tokens)
• Implements login page, protected route layout, dashboard, and a repo list on the home page to verify API access

Setup

1. Create a GitHub OAuth App (callback: http://localhost:3000/api/auth/callback/github)
2. Create D1 database: npx wrangler d1 create quickhub-db and update database_id in wrangler.jsonc
3. Copy .dev.vars.example to .dev.vars and fill in credentials
4. Apply migration: npx wrangler d1 migrations apply quickhub-db --local