feat(a11y): [ELUSoC_2026] add modal focus trap hook & shortcut manager

frontend/src/components/editor/QualityBadge.css

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+.quality-score-badge {
+  display: inline-block;
+  padding: 4px 8px;
+  border-radius: 4px;
+  font-size: 11px;
+  font-weight: 600;
+  color: white;
+}
+
+.quality-score-badge.high {
+  background: #10b981;
+}
+
+.quality-score-badge.medium {
+  background: #f59e0b;
+}

frontend/src/components/editor/QualityBadge.jsx

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+import React from 'react';
+import './QualityBadge.css';
+
+export default function QualityBadge({ score }) {
+  if (score === undefined || score === null) return null;
+
+  return (
+    <div className={`quality-score-badge ${score > 80 ? 'high' : 'medium'}`}>
+      Matting Score: {score}%
+    </div>
+  );
+}

python-ai-service/app/services/matting_quality_evaluator.py

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+"""
+matting_quality_evaluator.py — AI Background Matting Quality Metrics Evaluator
+Built for ELUSoC 2026 / GSSOC 2026.
+"""
+import numpy as np
+
+def evaluate_alpha_matting_quality(alpha_channel: np.ndarray) -> float:
+    if alpha_channel is None or alpha_channel.size == 0:
+        return 0.0
+
+    # Calculate boundary smoothness and edge sharpness
+    gradient_magnitude = np.abs(np.gradient(alpha_channel.astype(float)))
+    mean_gradient = float(np.mean(gradient_magnitude))
+    quality_score = min(100.0, max(0.0, (1.0 - mean_gradient / 255.0) * 100.0))
+    return round(quality_score, 2)

python-ai-service/app/services/test_matting_quality.py

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+"""
+test_matting_quality.py — Matting Quality Evaluator Tests
+Built for ELUSoC 2026 / GSSOC 2026.
+"""
+import numpy as np
+from app.services.matting_quality_evaluator import evaluate_alpha_matting_quality
+
+def test_alpha_matting_quality():
+    mock_alpha = np.ones((100, 100), dtype=np.uint8) * 255
+    score = evaluate_alpha_matting_quality(mock_alpha)
+    assert score >= 90.0