feat: add account management, email verification, AI fixes, and user guides

- Profile settings, account transfer, delete/leave account flows
- Email verification with JWT tokens and Resend integration
- AI assistant/copilot fixes: markdown rendering, shared RAG helpers,
  token tracking, input refocus, model_validate usage
- User guides hub + detail pages with 13 topic guides
- Sidebar and top bar navigation for guides

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

backend/app/api/endpoints/assistant_chat.py

@@ -198,7 +198,7 @@ async def post_message(
 
     return ChatMessageResponse(
         content=ai_content,
-        suggested_flows=[SuggestedFlow(**sf) for sf in suggested_flows],
+        suggested_flows=[SuggestedFlow.model_validate(sf) for sf in suggested_flows],
     )