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

app/plugins/builtins/ai_proactive/plugin.py

@@ -45,8 +45,12 @@ class AIProactivePlugin(BasePlugin):
         is_core=False,
     )
 
+    def __init__(self) -> None:
+        super().__init__()
+        self._proactive_handler = None
+
     async def on_activate(self, db, service_container, event_bus) -> None:
-        """Register context tools and subscribe to events."""
+        """Register context tools, subscribe to events, and register as participant."""
         await super().on_activate(db, service_container, event_bus)
         try:
             from app.plugins.builtins.ai_proactive.context_tools import (
@@ -61,8 +65,36 @@ class AIProactivePlugin(BasePlugin):
         except Exception:
             logger.exception("Failed to register AI Proactive context tools")
 
+        # Register as participant in the kommunikation system
+        try:
+            from app.plugins.builtins.ai_proactive.participant_handler import (
+                AIProactiveParticipantHandler,
+            )
+            from app.plugins.builtins.kommunikation.participant_registry import (
+                get_participant_registry,
+            )
+
+            self._proactive_handler = AIProactiveParticipantHandler(service_container)
+            get_participant_registry().register("ai_proactive", self._proactive_handler)
+            logger.info("AI Proactive registered as participant 'ai_proactive'")
+        except Exception:
+            logger.exception("Failed to register AI Proactive as participant")
+
     async def on_deactivate(self, db, service_container, event_bus) -> None:
-        """Unregister tools and event listeners."""
+        """Unregister tools, event listeners, and participant."""
+        # Unregister from participant registry
+        try:
+            from app.plugins.builtins.kommunikation.participant_registry import (
+                get_participant_registry,
+            )
+
+            get_participant_registry().unregister("ai_proactive")
+            logger.info("AI Proactive unregistered as participant 'ai_proactive'")
+        except Exception:
+            logger.exception("Failed to unregister AI Proactive as participant")
+
+        self._proactive_handler = None
+
         try:
             from app.plugins.builtins.ai_assistant.tool_registry import (
                 get_tool_registry,