fix: merge nearby spine gaps + handle multi-page crop in frontend

Backend: merge gaps within 5% of image width — the spine area may have
thin ink strips splitting one physical gap into multiple detected gaps.
Only use gaps >= 2% width as split points.

Frontend: StepCrop now handles multi_page crop responses without
crashing on missing original_size/cropped_size fields.

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

klausur-service/backend/page_crop.py

@@ -100,12 +100,33 @@ def detect_page_splits(
     if not gaps:
         return []
 
+    # Merge nearby gaps (< 5% of width apart) — the spine area may have
+    # thin ink strips between multiple gap segments
+    merge_dist = max(20, int(w * 0.05))
+    merged: list = [gaps[0]]
+    for g in gaps[1:]:
+        prev = merged[-1]
+        prev_end = prev["x"] + prev["width"]
+        if g["x"] - prev_end < merge_dist:
+            # Merge: extend previous gap to cover both
+            new_end = g["x"] + g["width"]
+            prev["width"] = new_end - prev["x"]
+            prev["center"] = prev["x"] + prev["width"] // 2
+        else:
+            merged.append(g)
+    gaps = merged
+
     # Sort gaps by width (largest = most likely spine)
     gaps.sort(key=lambda g: g["width"], reverse=True)
 
-    # Use the widest gap(s) as split points
-    # For now: support up to N-1 gaps → N pages
-    split_points = sorted(g["center"] for g in gaps[:3])  # max 4 pages
+    # Use only gaps that are significant (>= 2% of image width)
+    significant_gaps = [g for g in gaps if g["width"] >= w * 0.02]
+    if not significant_gaps:
+        # Fall back to widest gap
+        significant_gaps = [gaps[0]]
+
+    # Use the significant gap(s) as split points
+    split_points = sorted(g["center"] for g in significant_gaps[:3])
 
     # Build page rectangles
     pages: list = []