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

alembic/versions/0140_comm_conversation_folders.py

@@ -0,0 +1,28 @@
+"""Communication: folder_id in comm_conversations for folder organization
+
+Revision ID: 0140
+Revises: 0139
+Create Date: 2026-08-21
+
+Adds folder_id to comm_conversations for folder-based organization.
+"""
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+
+# revision identifiers, used by Alembic.
+revision = "0140"
+down_revision = "0139"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("comm_conversations", sa.Column("folder_id", PGUUID(as_uuid=True), nullable=True))
+    op.create_index("ix_comm_conversations_folder", "comm_conversations", ["folder_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_comm_conversations_folder", table_name="comm_conversations")
+    op.drop_column("comm_conversations", "folder_id")