feat: Unified Messaging System — kommunikation plugin, AI/Proactive/System participants, MessageSidebar, Rich Content Renderer

Phase 1: Backend plugin kommunikation (13 files, 10 tables, REST API, WebSocket, RBAC, DMS Bridge, Participant Registry, Mini-App Registry, Search Provider)
Phase 2: AI plugins as participants (ai_assistant + ai_proactive dock as participants, heartbeat job)
Phase 3: system_notif plugin (system events → chat messages, pinned System room)
Phase 4: Frontend MessageSidebar (replaces AISidebar, same design, comm API client, WebSocket hook, commStore)
Phase 5: Rich Content Block Renderer (11 components: Markdown, HTML, Image, Audio, Video, File, ActionCard, ContactCard, MiniApp, BlockRenderer)
BasePlugin: added services property + _container in on_activate

frontend/src/components/comm/blocks/BlockRenderer.tsx

@@ -0,0 +1,75 @@
+import React from 'react';
+import type { MessageBlock } from '@/store/commStore';
+import MarkdownBlock from './MarkdownBlock';
+import HtmlBlock from './HtmlBlock';
+import ImageBlock from './ImageBlock';
+import AudioBlock from './AudioBlock';
+import VideoBlock from './VideoBlock';
+import FileBlock from './FileBlock';
+import ActionCardBlock from './ActionCardBlock';
+import ContactCardBlock from './ContactCardBlock';
+import MiniAppBlock from './MiniAppBlock';
+
+interface BlockRendererProps {
+  blocks: MessageBlock[];
+}
+
+const BlockRenderer: React.FC<BlockRendererProps> = ({ blocks }) => {
+  if (!blocks || blocks.length === 0) {
+    return null;
+  }
+
+  // Sort blocks by sort_order to ensure correct rendering sequence
+  const sortedBlocks = [...blocks].sort((a, b) => a.sort_order - b.sort_order);
+
+  return (
+    <div className="space-y-2">
+      {sortedBlocks.map((block) => {
+        const renderBlock = (): React.ReactNode => {
+          switch (block.block_type) {
+            case 'text':
+              // Text blocks: render as plain text in a styled div
+              return (
+                <div className="text-sm whitespace-pre-wrap break-words">
+                  {block.block_data.text || block.block_data.content || ''}
+                </div>
+              );
+            case 'markdown':
+              return <MarkdownBlock block={block} />;
+            case 'html':
+              return <HtmlBlock block={block} />;
+            case 'image':
+              return <ImageBlock block={block} />;
+            case 'audio':
+              return <AudioBlock block={block} />;
+            case 'video':
+              return <VideoBlock block={block} />;
+            case 'file':
+              return <FileBlock block={block} />;
+            case 'action_card':
+              return <ActionCardBlock block={block} />;
+            case 'contact_card':
+              return <ContactCardBlock block={block} />;
+            case 'miniapp':
+              return <MiniAppBlock block={block} />;
+            default:
+              // Fallback for unknown block types
+              return (
+                <div className="text-xs text-secondary-400 italic p-2 rounded bg-secondary-50">
+                  Unbekannter Block-Typ: {block.block_type}
+                </div>
+              );
+          }
+        };
+
+        return (
+          <div key={block.id} className="block-wrapper">
+            {renderBlock()}
+          </div>
+        );
+      })}
+    </div>
+  );
+};
+
+export default BlockRenderer;