Polish export handoff workflow

backend/tests/test_sprint64_export_handoff_polish.py

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+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[2]
+
+
+def test_export_center_surfaces_handoff_readiness_context() -> None:
+    export_center = (ROOT / "frontend" / "src" / "components" / "exports" / "ExportCenter.tsx").read_text(
+        encoding="utf-8"
+    )
+
+    assert 'className="handoff-summary-card"' in export_center
+    assert 'className="handoff-readiness-grid"' in export_center
+    assert 'className="handoff-action-grid"' in export_center
+    assert 'className="export-type-badge"' in export_center
+    assert "Latest handoff artifact" in export_center
+    assert "Project report" in export_center
+    assert "Detection run" in export_center
+    assert "Segmentation run" in export_center
+    assert "selectedDetectionRunId || 'none'" in export_center
+    assert "selectedSegmentationRunId || 'none'" in export_center
+
+
+def test_export_handoff_polish_has_responsive_styles() -> None:
+    css = (ROOT / "frontend" / "src" / "styles" / "app.css").read_text(encoding="utf-8")
+
+    assert ".handoff-summary-card" in css
+    assert ".handoff-readiness-grid" in css
+    assert ".handoff-action-grid" in css
+    assert ".export-type-badge" in css
+    assert ".handoff-action-card" in css
+    assert "grid-template-columns: repeat(4, minmax(0, 1fr));" in css
+    assert ".handoff-action-grid" in css and "@media (max-width: 980px)" in css