fix: network diagram team_id guard + multi-style edge routing

Backend:
- Guard create_diagram with 422 if current_user.team_id is None (prevents
  NOT NULL constraint crash for accounts not yet assigned to a team)
- Add routing field to DiagramEdge schema (straight/curved/step)

Frontend:
- ConnectionEdge now supports straight (default), curved (bezier), and
  step (smooth-step) routing per-edge via routing field in edge data
- PropertiesPanel Connection section gets a Line Style toggle:
  Straight | Curved | Step buttons, active state highlights in accent
- handleEdgeUpdate and serializeEdges now propagate the routing field
- DiagramEdge type gets optional routing field

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

backend/app/api/endpoints/network_diagrams.py

@@ -130,6 +130,11 @@ async def create_diagram(
     db: Annotated[AsyncSession, Depends(get_db)],
     current_user: Annotated[User, Depends(get_current_active_user)],
 ) -> NetworkDiagramResponse:
+    if current_user.team_id is None:
+        raise HTTPException(
+            status_code=422,
+            detail="Network Diagrams require a team account. Assign your account to a team first.",
+        )
     diagram = NetworkDiagram(
         team_id=current_user.team_id,
         name=data.name,