docs: add ARCHITECTURE.md & ARCHITECTURE_QUICK.md, fix user pre bg

- ARCHITECTURE.md: comprehensive project documentation (868 lines)
  covering all 21 chapters (data model, services, endpoints,
  chat pipeline, tool system, agents, memory, skills, telegram,
  cronjobs, krea, auth, workspace, deployment, config)
- ARCHITECTURE_QUICK.md: compact quick-reference (223 lines)
  for fast lookups during development
- chat.css: add .msg.user.markdown pre selector so code blocks
  in user messages get correct background color (was only on code)

wwwroot/css/chat.css

@@ -103,6 +103,7 @@
   border: var(--border-width) solid var(--f3-border);
 }
 
+.msg.user.markdown pre,
 .msg.user.markdown code {
   background: var(--f2-bg);
 }