Touch is a panel setting, and the rail grows with it

It described the wrong object. A dashboard is a document that may hang on a
hallway tablet and in a desk browser at the same time, and only one of those
has fingers on it — so the flag moves off `DashboardDef.settings` and onto
`PanelDef` as a plain bool, ticked in the Panels dialog. `useCanvasRoot` takes
it as an argument rather than reading the document, and `/panel/{id}` is the
only surface with a panel to ask.

Dropping the message binding with it is deliberate: nothing drove it, and a
flow deciding whether a screen has fingers on it was never the point. A stored
`settings.touch` is inert rather than migrated, which `_check_settings`
skipping unknown names already guaranteed.

The rail was the other half. It had no touch behaviour at all and its 40px
buttons met neither branch of the 44/32 rule. `[data-touch] .dui-rail{-item}`
in `ui/core/core.css` spends the padding and the gap on the buttons instead,
so they reach the 44px target and the rail comes out taller at exactly the
same width — `RAIL_INSET` never moves, and the arrangement under it does not
either.

Also closes the panels-dialog icon gap: `DashboardSummary` carries the `icon`
now, so the dialog draws each assigned dashboard's rail glyph beside its
checkbox. `initials()` went from three identical copies in the looks to one in
`Dashboard/icons.ts`, so the dialog and the rail fall back the same way.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Va7ExQDtuwKN7kNpHhWWNQ

frontend/tests/widgets.spec.ts

@@ -620,37 +620,6 @@ test("a chart's cursor follows the pointer", async ({ page }) => {
   ).toBeLessThan(3)
 })
 
-test("touch makes the controls bigger without changing what they do", async ({
-  page,
-}) => {
-  await openPanel(page)
-  const control = page.getByTestId("widget-frame").filter({ hasText: "Mode" })
-  const pointer = (await control.getByRole("button").first().boundingBox())!
-    .height
-
-  await setLook(page, dashboardName, { touch: { value: true } })
-  await openPanel(page)
-  await expect(page.getByTestId("canvas-surface")).toHaveAttribute(
-    "data-touch",
-    "",
-  )
-  const touched = (await control.getByRole("button").first().boundingBox())!
-    .height
-  expect(
-    touched,
-    `a segment is ${touched.toFixed(1)} touched and ${pointer.toFixed(1)} pointed at`,
-  ).toBeGreaterThan(pointer)
-
-  // The control is the same control: it still publishes what it always did.
-  await control.getByRole("button", { name: "Boost" }).click()
-  await expect(control.getByRole("button", { name: "Boost" })).toHaveAttribute(
-    "aria-pressed",
-    "true",
-  )
-
-  await setLook(page, dashboardName, { touch: { value: false } })
-})
-
 /**
  * What a dashboard was told to wear, as a wall panel would be told.
  *