Marks as dots inside the track: ticks, a selector's options and a slider's reading on the track's midline and the ring's centre line, coloured by the fill or the handle over them; the selector ring fills to its option

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/tests/widgets.spec.ts

@@ -432,8 +432,10 @@ test("a slider writes what it sets against under its value", async ({
   await expect(page.getByTestId("slider-reading")).toHaveText(
     /^Level\s*80\.0\s*kW$/,
   )
-  // And placed on the track, as a mark beside the handle.
-  await expect(page.locator(".dui-slider-mark")).toBeVisible()
+  // And placed on the track, as a dot inside it.
+  await expect(
+    page.locator(".dui-slider-dots > .dui-slider-reading"),
+  ).toBeVisible()
 })
 
 test("a reading is written to its own precision while it is moving", async ({