fix(layer-list): title visibility button with the action it performs (#2135)

- Fixes #1675

## Problem

In the layer list, the show/hide button's `title` tooltip named the
layer's
current visibility instead of what a click does. A visible layer's
button read
`show` while clicking it hides the layer, and a hidden layer's button
read
`hide` while clicking it shows the layer — exactly reversed.

`LayerListItem.tsx` derived a single value from the layer state:

```ts
const visibilityAction = visibility === "visible" ? "show" : "hide";
```

and passed it to `IconAction` as `action`, which `IconAction` used for
three
different things: choosing the icon, building the `--show`/`--hide` CSS
modifier, and rendering `title={this.props.action}`. The first two are
correct
as a state indicator; only the tooltip needs the opposite verb.

## Change

- `IconAction` gets an optional `title` prop and falls back to `action`
when it
  is not given, so the delete and duplicate buttons are unchanged.
- `LayerListItem` keeps `visibilityAction` (icon +
`maputnik-layer-list-icon-action__visibility--hide`
CSS modifier, which `_layer.scss` relies on to keep the button visible
for
  hidden layers) and adds `visibilityTitle` for the inverted tooltip.

No icon, class name or click behaviour changes.

## Test

New regression test in `e2e/layers-list.spec.ts`: it asserts the button
is
titled `hide` while the layer is visible, clicks it, and asserts it is
titled
`show` once the layer is hidden. Against the unpatched component the
first
assertion fails with `Expected: "hide"` / `Received: "show"`,
reproducing the
reported behaviour.

`npm run lint`, `npx tsc --noEmit`, `npx vitest run` (50 tests) and
`npx playwright test e2e/layers-list.spec.ts` (28 tests) all pass.

## Note

The tooltips in this component (`delete`, `duplicate`, `show`, `hide`)
are
plain English literals and are not run through `t()` today, so this
change
keeps them as-is rather than introducing translation keys for one
button. Also
worth flagging separately: these buttons carry `aria-hidden="true"`, so
the
`title` never reaches assistive technology at all — out of scope here,
but it
means the label is a mouse-hover tooltip only.

e2e/layers-list.spec.ts

@@ -1,4 +1,4 @@
-import { beforeEach, describe, test } from "./utils/fixtures";
+import { beforeEach, describe, expect, test } from "./utils/fixtures";
 import { MaputnikDriver } from "./maputnik-driver";
 
 describe("layers list", () => {
@@ -47,6 +47,14 @@ describe("layers list", () => {
       });
     });
 
+    test("should title the visibility button with the action it performs", async () => {
+      const key = "layer-list-item:" + id + ":toggle-visibility";
+      // A visible layer is about to be hidden, and a hidden one to be shown.
+      await expect(get.elementByTestId(key)).toHaveAttribute("title", "hide");
+      await when.click(key);
+      await expect(get.elementByTestId(key)).toHaveAttribute("title", "show");
+    });
+
     describe("when clicking hide", () => {
       beforeEach(async () => {
         await when.click("layer-list-item:" + id + ":toggle-visibility");