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

app/plugins/builtins/kommunikation/models.py

@@ -52,6 +52,7 @@ class CommConversation(Base, TenantMixin, OwnedMixin):
     last_msg_preview: Mapped[str | None] = mapped_column(Text, nullable=True)
     last_msg_sender_type: Mapped[str | None] = mapped_column(String(50), nullable=True)
     metadata_: Mapped[dict[str, Any]] = mapped_column("metadata", JSONB, default=dict, nullable=False)
+    folder_id: Mapped[uuid.UUID | None] = mapped_column(PGUUID(as_uuid=True), nullable=True)
 
 
 class CommParticipant(Base, TenantMixin, OwnedMixin):