feat: add beans serve command with web UI

[hmans]

Summary

This PR adds a beans serve command that starts a web server providing a full-featured web UI for browsing and managing beans.

• Web UI - A SvelteKit SPA for viewing beans with hierarchical display, markdown rendering, and syntax-highlighted code blocks
• GraphQL API - HTTP endpoint at /api/graphql exposing queries, mutations, and subscriptions
• Live Updates - Real-time sync via GraphQL subscriptions over WebSockets

Key Features

Backend (beans serve)

• HTTP server using Gin framework
• GraphQL subscriptions for real-time bean change notifications
• Static assets embedded into Go binary via //go:embed
• Configurable port: CLI --port flag, config file server.port, or default (8080)
• Graceful shutdown with proper signal handling
• Request logging middleware

Frontend (SvelteKit)

• Two-pane layout: hierarchical bean list on the left, bean detail on the right
• Nested display respecting parent/child relationships
• Markdown rendering for bean bodies with styled headings and task lists
• Shiki syntax highlighting for code blocks (bundled essential languages only)
• Copy bean ID to clipboard button
• Real-time updates via GraphQL subscription
• Svelte 5 runes-style reactive store

GraphQL Enhancements

• New Subscription type with beanChanged field
• includeInitial parameter to receive existing beans on subscription start
• INITIAL_SYNC_COMPLETE event for explicit sync signaling (no magic timeouts)
• BeanChangeEvent type with INITIAL, INITIAL_SYNC_COMPLETE, CREATED, UPDATED, DELETED change types

Configuration

• New server.port option in .beans.yml (default: 8080)
• beans init now includes server config in generated file
• CLI --port flag overrides config value

Architecture

beans serve
    ├── /api/graphql  → GraphQL endpoint (queries, mutations, subscriptions)
    ├── /playground   → GraphQL Playground
    └── /*            → SvelteKit SPA (embedded static assets)

Usage

# Start server on default port (8080)
beans serve

# Start on custom port
beans serve --port 3000

# Or configure in .beans.yml:
# server:
#   port: 3000

Development Workflow

# Start both backend and frontend with hot reload
mise dev

# Or manually:
# Terminal 1: Backend
go run . serve

# Terminal 2: Frontend dev server (proxies /api/graphql to backend)
cd frontend && pnpm dev

Test Plan

• Run beans serve and verify the web UI loads at http://localhost:8080
• Verify beans are displayed in hierarchical order
• Click a bean and verify the detail view shows markdown-rendered body
• Create/update/delete a bean via CLI and verify real-time updates in the UI
• Verify beans server alias works
• Verify --port flag overrides default
• Verify server.port in config is respected
• All tests pass