Add: Agent v2.1.2, WebSocket Live Shell, WebRTC Remote Desktop (Beta), Announcement Push

- Agent v2.1.2: WebSocket ShellService, ACK nach WS-Ankündigungen, shown_announcements Fix
- Backend: shellServer.js mit WebSocket-Server (Shell + Announcement Push + RTC Signaling)
- Backend: patch.controller.js Fix (command_id=0 Falsy-Bug beim Auto-Update)
- Frontend: Remote Desktop Tab (WebRTC Beta) in AgentDetailPage für super_admin/admin
- Frontend: PatchManagementPage auf v2.1.2 aktualisiert
- WPF Notification: AllowsTransparency=False + kein DropShadowEffect (Dispatcher-Crash Fix)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/routes/patch.routes.js

@@ -26,4 +26,9 @@ router.post('/commands/trigger',     requireAdmin, ctrl.triggerCommand);
 router.post('/commands/trigger-group', requireAdmin, ctrl.triggerGroupCommand);
 // (result route is before authenticateToken above)
 
+// Remote Shell
+router.post('/shell/:agentId',              requireAdmin, ctrl.triggerShell);
+router.get('/shell/:agentId/result/:cmdId', requireAdmin, ctrl.getShellResult);
+router.get('/shell/:agentId/history',       requireAdmin, ctrl.getShellHistory);
+
 module.exports = router;