Clean up legacy agent code and improve UI progress indicators

- Remove legacy Foundry SDK agents (base_agent.py, simple_agent.py, triage_agent.py, planning_agent.py, research_agent.py, text_content_agent.py, compliance_agent.py)
- Keep only image_content_agent.py with generate_dalle_image function
- Update agents/__init__.py to export only generate_dalle_image
- Remove _AzureAISettings from settings.py (no longer needed)
- Add progress status indicators for all API operations:
  - 'Parsing creative brief...' for initial brief parsing
  - 'Updating creative brief...' for brief refinements
  - 'Finding products...' for product selection
  - 'Processing your request...' for general chat
- Fix loading spinner to show during all loading states
- Update frontend proxy server.js:
  - Add 5-minute timeout for long-running requests
  - Add streaming support headers for SSE responses
  - Improve error handling for proxy errors

Author: hunterjam

content-gen/src/backend/agents/__init__.py

@@ -1,7 +1,11 @@
-"""Agents package for Content Generation Solution Accelerator."""
+"""Agents package for Content Generation Solution Accelerator.
 
-from backend.agents.simple_agent import SimpleAgent
+The multi-agent workflow is handled by the orchestrator using Microsoft Agent Framework.
+This package provides utility functions used by the orchestrator.
+"""
+
+from backend.agents.image_content_agent import generate_dalle_image
 
 __all__ = [
-    "SimpleAgent",
+    "generate_dalle_image",
 ]