Webapp: Design system, auth, and real-time chat

The webapp foundation ships with a design system, authentication, real-time chat, and dark mode.

• Three-column layout: conversations, active chat, and details
• Glassmorphism effects with full dark mode support
• Email authentication with session persistence
• Real-time messages via WebSocket, iMessage-style bubble layout
• Rich text composer: bold, italic, lists, links
• Drag-and-drop file uploads with progress bar and inline preview
• Cursor-based pagination for conversation lists
• English and Spanish localization