supports-edges draw toggle — last row of the graph settings' first section

Off hides supports edges from the canvas: the renderer skips them when
building the shaft/head paths (draw-only — the physics sim still feels
them and detail cards still list them; settings never shift the graph).
Server-persisted with the rest (render-settings.json); sanitizer takes
real booleans only, anything else (missing key, legacy file) loads ON —
today's look. Toggle row styled as the Background-colors row; sanitiser
pinned in render-settings.test.

tests/render-settings.test.ts

@@ -118,6 +118,7 @@ describe("sanitizeRenderSettings", () => {
 			arrowLength: 3,
 			labelZoom: 2,
 			labelSize: 4,
+			showSupports: false,
 			clusterLabels: false,
 			clusterLabelSize: 20,
 			regions: DEFAULT_REGION_TUNING,
@@ -126,6 +127,16 @@ describe("sanitizeRenderSettings", () => {
 		assert.deepEqual(sanitizeRenderSettings(valid), valid); // every value a ladder member
 	});
 
+	test("showSupports (2026-09-29): boolean-only, default on — the draw toggle", () => {
+		// A render-only hide for supports edges: anything but a real boolean
+		// (missing key, legacy file, garbage) loads as ON (today's look); a
+		// real false round-trips untouched.
+		assert.equal(sanitizeRenderSettings({}).showSupports, true);
+		assert.equal(sanitizeRenderSettings({ showSupports: false }).showSupports, false);
+		assert.equal(sanitizeRenderSettings({ showSupports: "no" }).showSupports, true);
+		assert.equal(sanitizeRenderSettings({ showSupports: 0 }).showSupports, true);
+	});
+
 	test("the pre-2026-09-13 stored shape (flat, no regions/physics) still loads", () => {
 		// data/render-settings.json on disk predates the nested sections —
 		// those loads must get defaults, not crash, not shift anything.