Fix BOM breaking bin/*.bat and add early Graphviz check to --tosvg wrappers

setenv.bat (and activate_venv/get_cmd/install_py.bat) had a UTF-8 BOM,
which cmd.exe read as part of the @echo off line, breaking every bin
script that calls setenv.bat via `call`. Stripped the BOM.

material_flow.bat/.sh and tro_flow.bat/.sh now check up front whether
the Graphviz engine --tosvg needs (dot, or neato with --use-cords) is
resolvable, and print an early WARNUNG with fix instructions if not -
before the CSV is even parsed. The Python scripts still run and report
the same failure in detail if the SVG step is actually reached.

bin/material_flow.sh

@@ -28,4 +28,24 @@ fi
 PY="python3"
 command -v python3 >/dev/null 2>&1 || PY="python"
 
+# Graphviz pruefen, wenn --tosvg SVG erzeugen soll (dot, bei --use-cords/
+# --use-coords neato) - nur ein frueher Hinweis; das eigentliche Skript
+# meldet denselben Fehler beim Rendern noch einmal im Detail.
+case " $* " in
+    *" --tosvg "*)
+        GV_ENGINE="dot"
+        case " $* " in
+            *" --use-cords "*|*" --use-coords "*) GV_ENGINE="neato" ;;
+        esac
+        if ! command -v "$GV_ENGINE" >/dev/null 2>&1; then
+            if [ "$GV_ENGINE" != "dot" ] || [ -z "${GRAPHVIZ_DOT:-}" ]; then
+                echo "WARNUNG: Graphviz '$GV_ENGINE' nicht im PATH gefunden - --tosvg wird vermutlich fehlschlagen." >&2
+                echo "         Graphviz installieren (https://graphviz.org/download/), PATH ergaenzen," >&2
+                echo "         oder GRAPHVIZ_DOT auf die dot-Programmdatei setzen (nur fuer 'dot')." >&2
+                echo "" >&2
+            fi
+        fi
+        ;;
+esac
+
 exec "$PY" "$SKEL_LIB/material_flow.py" "$@"