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>

src/multitenancy/credential-store.ts

@@ -23,7 +23,7 @@ function decrypt(ciphertext: string): string {
   return decipher.update(encrypted) + decipher.final('utf8');
 }
 
-export type Platform = 'email' | 'whatsapp' | 'linkedin' | 'telegram' | 'discord' | 'instagram' | 'twitter' | 'tiktok' | 'snapchat' | 'facebook' | 'obsidian';
+export type Platform = 'email' | 'whatsapp' | 'linkedin' | 'telegram' | 'discord' | 'instagram' | 'twitter' | 'tiktok' | 'snapchat' | 'facebook' | 'obsidian' | 'slack';
 
 export interface EmailCredentials {
   host: string;