Polish export and system mobile layout

backend/tests/test_sprint76_export_system_mobile_polish.py

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+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[2]
+
+
+def test_export_and_system_mobile_density_css_contracts() -> None:
+    css = (ROOT / "frontend" / "src" / "styles" / "app.css").read_text(encoding="utf-8")
+
+    assert ".system-provider-list" in css
+    assert ".system-provider-card" in css
+    assert ".system-provider-grid" in css
+    assert ".system-provider-card div" in css
+    assert ".provider-layer-list" in css
+    assert ".provider-layer-chip" in css
+    assert ".export-action-grid" in css
+    assert ".handoff-action-card p" in css
+    assert ".export-history-controls input" in css
+    assert ".export-history-controls select" in css
+    assert ".export-card-header" in css
+    assert "minmax(7.5rem, 1fr)" in css
+    assert "overflow-wrap: anywhere;" in css
+
+
+def test_export_and_provider_components_keep_workflow_markup() -> None:
+    export_center = (ROOT / "frontend" / "src" / "components" / "exports" / "ExportCenter.tsx").read_text(
+        encoding="utf-8"
+    )
+    provider_panel = (ROOT / "frontend" / "src" / "components" / "providers" / "ProviderPanel.tsx").read_text(
+        encoding="utf-8"
+    )
+
+    assert 'className="export-center"' in export_center
+    assert "export-action-grid" in export_center
+    assert "handoff-action-card" in export_center
+    assert "export-history-controls" in export_center
+    assert "export-card-header" in export_center
+    assert "system-provider-list" in provider_panel
+    assert "system-provider-card" in provider_panel
+    assert "system-provider-grid" in provider_panel
+    assert "provider-layer-chip" in provider_panel