feat(chat): redesign attachments with ContentBlock format and file downloads (#397)

- Redesign attachment handling using Anthropic-style ContentBlock array format
  with discriminated unions (text, image, file types)
- Add frontend file download functionality supporting both ContentBlock
  and Markdown formats with authentication tokens
- Fix multi-process conflict causing SQLite database resets by eliminating
  redundant nodemon instances
- Update chat store to build ContentBlock arrays from attachments
- Improve image handling with base64 conversion for upstream API

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "hermes-web-ui",
-  "version": "0.5.5",
+  "version": "0.5.6",
   "description": "Self-hosted AI chat dashboard for Hermes Agent — multi-model (Claude, GPT, Gemini, DeepSeek) web UI with Telegram, Discord, Slack, WhatsApp integration",
   "repository": {
     "type": "git",