Community-specific logos/icons in chat widget

[neuromechanist]

Summary

Replace the hardcoded brain icon in the chat widget header with community-specific logos. Each community should be able to provide their own icon/logo that displays in the widget avatar area.

Logo Resolution (Precedence, highest to lowest)

1. Widget JS config (setConfig({ logo: '...' })) - Embedder override, highest precedence
2. YAML config (widget.logo_url field) - Community maintainer sets in config.yaml
3. Convention-based - A logo.png or logo.svg file in the community's folder (src/assistants/{id}/)

Implementation Plan

Backend

• Add logo_url field to WidgetConfig in src/core/config/community.py
• Add API endpoint to serve community logo files (e.g., GET /{community_id}/logo)
• During config loading, auto-detect logo.png/logo.svg in community folder if no explicit logo_url set
• Return logo URL in community config API response

Frontend (Widget)

• Update applyWidgetConfig() to handle logo from API response
• Support logo in setConfig() for embedder override
• Render as <img> when a URL is provided, fall back to brain SVG icon
• Proper sizing/scaling within the existing avatar circle

Config Schema

widget:
  title: HED Assistant
  logo_url: https://example.com/hed-logo.png  # Optional
  # ... existing fields

Acceptance Criteria

• Communities can specify logo via YAML config
• Embedders can override logo via widget JS config
• Convention-based logo detection (file in folder)
• Graceful fallback to brain icon when no logo provided
• Logo properly scaled within avatar circle
• Works with both PNG and SVG formats