feat: MVP UI Overhaul, Bun Migration, Better Auth integration, and Mobile Optimization

[VDuda]

Summary

This PR represents a comprehensive overhaul of the OpenBookLM codebase to stabilize the MVP state, migrate to modern tooling, and align the UI/UX with the latest design mockups.

Key Changes

• Migration to Bun: Switched from pnpm to bun for significantly faster installation and execution.
• Better Auth Integration:
  • Fully replaced Clerk with Better Auth.
  • Implemented a sleek Login Modal supporting GitHub, Google, and Magic Links.
  • Resolved Prisma schema constraints for magic link verification.
• Persistent Guest Mode:
  • Implemented cookie-based session persistence for guest users.
  • Guest users now retain their notebooks and credits for up to 7 days without needing to log in.
• UI/UX Overhaul:
  • Home Page: Updated to a clean, grid-based layout with a dedicated 'Create new notebook' card.
  • Mobile Optimization: Implemented a responsive List View for mobile and converted modals into 'Bottom Sheets' that slide up from the footer.
  • Unified Add Source Modal: Built a beautiful, unified dialog for adding various source types (Web, Drive, Files).
  • Studio Redesign: Updated the Studio sidebar to a 9-grid layout matching design mocks.
• Open Source Engagement:
  • Added a Not Implemented modal to all placeholder buttons, encouraging community contributions by linking directly to GitHub Issues.
  • Created a comprehensive CONTRIBUTING.md guide.
• Stability & Cleanup:
  • Standardized all API routes to return strict JSON (fixing 'Unexpected token U' errors).
  • Fixed Docker build issues related to Bun lockfile versions and user permissions.
  • Removed outdated Clerk scripts and redundant Kubernetes manifests.

Verification

• Verified production build via bun run build.
• Successfully built both Frontend and Backend Docker images locally.
• Manually tested Guest Mode persistence and Auth flow.