Refactor UI to use only shadcn components (#75)

Remove custom component files and inline all UI logic into App.tsx using only shadcn components (Card, Dialog, Badge, Button, Input, ScrollArea). This simplifies the component structure while maintaining all functionality including session browsing, message viewing, and search.

Changes:
- Deleted custom components: Message, SearchModal, SessionDetail, SessionItem, SessionList
- Refactored App.tsx to be self-contained with shadcn components
- Kept existing hooks (useSessions, useSessionDetail, useSearch)
- Applied Biome formatting and linting

Co-authored-by: Claude <noreply@anthropic.com>

Author: sanggggg