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>

packages/client/src/api/hermes/chat.ts

@@ -1,13 +1,18 @@
 import { io, type Socket } from 'socket.io-client'
 import { request, getBaseUrlValue, getApiKey } from '../client'
 
+export type ContentBlock =
+  | { type: 'text'; text: string }
+  | { type: 'image'; name: string; path: string; media_type: string }
+  | { type: 'file'; name: string; path: string; media_type?: string }
+
 export interface ChatMessage {
   role: 'user' | 'assistant' | 'system'
-  content: string
+  content: string | ContentBlock[]
 }
 
 export interface StartRunRequest {
-  input: string | ChatMessage[]
+  input: string | ContentBlock[]
   instructions?: string
   session_id?: string
   model?: string