feat: migrate legacy chat UI into console React app

[maxnoller]

Summary

• Ports the legacy docs/chat.html (4,066-line vanilla JS/CSS/HTML) into the console React app as a first-class /chat route
• Full feature parity: streaming responses (NDJSON + RAF scheduling), markdown rendering, message editing/branching, file uploads, slash commands, approval workflows, artifacts, responsive sidebar
• Shares the console's theme (dark mode included), auth, and component infrastructure — zero new dependencies, zero backend changes

Architecture

console/src/
├── api/chat-types.ts          # TypeScript interfaces for all chat API shapes
├── api/chat.ts                # API client (reuses requestJson from client.ts)
├── lib/chat-helpers.ts        # Pure utilities: session IDs, clipboard, approval commands
├── lib/chat-stream.ts         # NDJSON streaming fetch with 401 handling
├── lib/markdown.ts            # Lightweight markdown-to-HTML (ported from legacy)
└── routes/chat/
    ├── index.ts               # Barrel re-export
    ├── chat-page.tsx           # Page orchestrator (~380 lines)
    ├── use-chat-stream.ts     # Streaming hook: RAF render loop, AbortController
    ├── message-block.tsx      # MessageBlock (memo'd), ArtifactCard, EditInline
    ├── composer.tsx            # Composer + SlashSuggestions
    ├── chat-sidebar.tsx       # Session list sidebar
    └── chat-page.module.css   # All styles using console CSS custom properties

No single file exceeds 400 lines. Pure utilities separated from React for testability.

Test plan

• Navigate to /admin/chat — verify the chat UI renders with sidebar and empty state
• Send a message — verify streaming response renders incrementally with markdown
• Test markdown: headings, code blocks, lists, tables, bold/italic, links, blockquotes
• Attach file via button and paste — verify pending chips and send
• Edit a user message — verify branch creation and re-send
• Test approval workflow buttons (Allow once/always/session/agent/all, Deny)
• Type / — verify slash suggestions with keyboard nav
• Copy and regenerate message action buttons
• Resize below 900px — verify mobile sidebar overlay
• Toggle dark mode — verify all elements use theme variables
• Switch between sessions in sidebar
• Verify 401 errors redirect to login
• npx tsc --noEmit passes
• npm test passes (all 79 existing tests, zero regressions)

🤖 Generated with Claude Code