Slider reading in a tile: a line under the value in every look, row and fader; Beside it on every dashboard; tile geometry in the docs

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

frontend/tests/widgets.spec.ts

@@ -249,6 +249,21 @@ test.beforeAll(async ({ browser }) => {
         unit: " kW",
       },
     },
+    {
+      id: "against",
+      type: "slider",
+      title: "Against",
+      layout: { lg: { x: 6, y: 0, w: 4, h: 2 } },
+      config: {
+        target: w("setpoint"),
+        dtype: "float",
+        min: 0,
+        max: 100,
+        step: 0.5,
+        unit: " kW",
+        reading: { message: w("level"), dtype: "float", label: "Level" },
+      },
+    },
   ]
   const stacked = await (
     await api(page, `/dashboards/${stackName}`, { method: "PUT", data: stack })
@@ -409,6 +424,16 @@ test("rows are drawn in the order they were configured", async ({ page }) => {
   await expect(rows.nth(2)).toContainText(/grid/i)
 })
 
+test("a slider writes what it sets against under its value", async ({
+  page,
+}) => {
+  await openPanel(page, stackName)
+  // In the slider's unit and to its step's decimals, not the reading's own.
+  await expect(page.getByTestId("slider-reading")).toHaveText(
+    /^Level\s*80\.0\s*kW$/,
+  )
+})
+
 test("a reading is written to its own precision while it is moving", async ({
   page,
 }) => {