feat: ChatSources sidebar with grouped source links

[PAMulligan]

Summary

• Added Source type (blog | page | external) to API types with optional sources field on ChatMessage and ChatResponse
• Built ChatSources slide-out sidebar component that groups sources by type (blogs first, then pages, then external) with count header and close button
• Built SourceIcon button with badge count that appears on assistant messages with sources
• Integrated sidebar into ChatWindow with open/close/toggle state management
• Wired useChat hook to pass sources from API responses through to messages

Test Plan

• 75 tests passing (22 new tests added across 4 test files)
• Source type tests verify all type variants and optional field behavior
• SourceIcon tests cover badge, tooltip, click handler, active/inactive states
• ChatSources tests cover grouping, ordering, count header, links, close button
• MessageBubble tests verify icon renders only for assistant messages with sources
• ChatWindow integration tests verify sidebar open/close/toggle behavior
• Visual verification with mock data in dev server

Closes #4

🤖 Generated with Claude Code

[cloudflare-workers-and-pages]

Deploying chat-widget with    Cloudflare Pages

Latest commit:	631ba9f
Status:	✅  Deploy successful!
Preview URL:	https://23d46634.chat-widget-ejc.pages.dev
Branch Preview URL:	https://4-source-and-search-results.chat-widget-ejc.pages.dev

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