feat(UI-Overhaul-Phase8): Kommunikation UI Verbesserungen

Migration 0140: Add folder_id column to comm_conversations for folder organization

Backend:
- CommConversation model: add folder_id field (nullable UUID)

Frontend:
- Communication.tsx: Wider tree panel (ResizablePanel initialWidth=320, minWidth=240, maxWidth=450)
- Phase 8.2 (AI Chat in Kommunikation) already completed in Phase 2

tsc clean, backend import OK

frontend/src/pages/Communication.tsx

@@ -831,9 +831,9 @@ export function CommunicationPage() {
       {/* Desktop: two-pane layout */}
       <div className="hidden md:flex w-full">
         <ResizablePanel
-          initialWidth={288}
-          minWidth={200}
-          maxWidth={400}
+          initialWidth={320}
+          minWidth={240}
+          maxWidth={450}
           className="border-r border-secondary-200 bg-white"
         >
           <ConversationTree