feat: add docs chat widget UI components (E3)

- Add useDocsChat hook with message state, streaming, error handling
- Add ChatMessages component with auto-scroll and streaming cursor
- Add ChatInput component with Enter/Shift+Enter support
- Add DocsChatWidget floating container with amber styling
- Add AI integration specs (PRD, FO, bouwplan)

Implements Epic 3 of AI Documentatie Assistent feature.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/docs-chat/index.ts

@@ -0,0 +1,5 @@
+export { DocsChatWidget } from './docs-chat-widget'
+export { ChatMessages } from './chat-messages'
+export { ChatInput } from './chat-input'
+export { useDocsChat } from './use-docs-chat'
+export type { ChatMessage } from './use-docs-chat'