feat: Slack platform + Claude-powered chat support widget

- Add Slack as customer-facing messaging platform (client, 4 MCP tools, dashboard card)
- Add /api/chat endpoint powered by Claude Haiku with SquareMCP system prompt
- Add embeddable chat-widget.js injected into all 3 sites (docs, app, www)
- Add ANTHROPIC_API_KEY, serve product/ as static files
- Update Platform type to include slack

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -18,6 +18,7 @@
     "test:watch": "vitest"
   },
   "dependencies": {
+    "@anthropic-ai/sdk": "^0.96.0",
     "@modelcontextprotocol/sdk": "^1.0.0",
     "@types/cors": "^2.8.19",
     "bcryptjs": "^3.0.3",