feat: add crop box edge labels (T/B/L/R) for orientation

- Shows which edge is Top, Bottom, Left, Right on crop rectangle
- Removes ambiguity about crop box orientation
- Green labels on all four edges for clear visual reference

frontend/components/DebugRotationPanel.tsx

@@ -213,6 +213,25 @@ export function DebugRotationPanel({
     ctx.shadowBlur = 3;
     ctx.strokeRect(cropX, cropY, cropW, cropH);
 
+    // Draw edge orientation labels on crop box
+    ctx.fillStyle = '#00FF00';
+    ctx.font = 'bold 12px monospace';
+    ctx.textAlign = 'center';
+    ctx.textBaseline = 'middle';
+    ctx.shadowColor = 'rgba(0, 0, 0, 0.8)';
+    ctx.shadowBlur = 2;
+
+    // Top edge label
+    ctx.fillText('T', cropX + cropW / 2, cropY - 8);
+    // Bottom edge label
+    ctx.fillText('B', cropX + cropW / 2, cropY + cropH + 8);
+    // Left edge label
+    ctx.textAlign = 'right';
+    ctx.fillText('L', cropX - 8, cropY + cropH / 2);
+    // Right edge label
+    ctx.textAlign = 'left';
+    ctx.fillText('R', cropX + cropW + 8, cropY + cropH / 2);
+
     // Draw resize handles at corners
     const handleSize = 8;
     ctx.fillStyle = '#FFB800';