Force interactive matplotlib backend in pick_barrier

Some environments default matplotlib to Agg (non-interactive), which
silently no-ops plt.show() — the picker would print "FigureCanvasAgg
is non-interactive" and never display the thumbnail grid. Probe TkAgg
> QtAgg > Qt5Agg > GTK3Agg before pyplot import.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/pick_barrier.py

@@ -41,6 +41,17 @@ import sys
 from pathlib import Path
 
 import cv2
+import matplotlib
+# Reason: force an interactive backend so plt.show() actually opens a window.
+# Some environments (depending on matplotlibrc / DISPLAY / installed Qt-vs-Tk
+# bindings) default to Agg, which silently no-ops the picker. Try the common
+# GUI backends in order; fall through to whatever is set if none are available.
+for _backend in ("TkAgg", "QtAgg", "Qt5Agg", "GTK3Agg"):
+    try:
+        matplotlib.use(_backend, force=True)
+        break
+    except (ImportError, ValueError):
+        continue
 import matplotlib.pyplot as plt
 import numpy as np
 import pandas as pd