fix(ui): the phone's context sheet says when it scrolls

The bottom sheet's body has scrolled since #60 and said nothing about
it. Measured at 424x439, the track list's menu ended at y=470 with the
fold at 439 — reachable, since the body is `overflow-y: auto`, but with
no affordance saying so, and worst where the cut lands on a row
boundary and the sheet ends in a clean edge that reads as the end of
the list.

The cap stays: `menu-surface`'s own comment says a surface covering the
whole screen is a page, not a sheet. What changes is that the body
draws a fade, from two background layers whose *attachments* are the
feature — a shadow pinned to the box (`scroll`) under a cover of the
sheet's own colour painted at the end of the content (`local`), which
scrolls up over the shadow exactly when there is nothing more to see.
So the fade is absent on a menu that fits, present the moment one does
not, and gone again at the end of the list, with no scroll listener and
nothing reaching into `wa-dialog`'s shadow root for the scroller.
`background-attachment` is Chrome 4; the reference device is Chrome 113.

The other two options in the report — a shortened last row, or a max
height that makes the cut obvious — both need `height mod 48`, which
CSS cannot express, and the observed case is exactly the one where the
cut already lands on a row boundary.

The curve is steep rather than linear because the rows under it stay
live: a scrim over a menu item is that item's text surface, so the
4.5:1 rule reaches it, and the light ramp is what makes that real.
A 48px linear scrim at 0.8 greyed the last label to 5.0:1; 32px already
down to a quarter strength at 14px measures 9.9:1 there and spends its
weight on the strip below it.

The test asserts the pair of attachments rather than the pixels, on
this file's existing grounds that no tier here renders like the device
— it fails on the pre-fix stylesheet with `expected 'scroll' to be
'local, scroll'`. The rendered result was measured in the harness and
is recorded in `.planning/NOTES.md`.

Closes #207

frontend/test/components/menu-surface.test.ts

@@ -206,6 +206,56 @@ describe('menu-surface', () => {
       expect(dismissed, 'no menu-dismiss reached the document').toBe(1);
     });
 
+    /**
+     * The scroll affordance (#207), and this is the mechanism again
+     * rather than the symptom.
+     *
+     * The sheet's body has scrolled since #60 and said nothing about
+     * it: measured at 424x439, eight items ended at y=470 with the
+     * fold at 439, and where the cut lands on a row boundary the sheet
+     * ends in a clean edge that reads as the end of the list.
+     *
+     * What makes the fade *conditional* — absent on a menu that fits,
+     * present the moment one does not, gone again at the end of the
+     * list — is `background-attachment`, not a scroll listener: a cover
+     * of the sheet's own colour is painted at the end of the content
+     * and attached `local`, over a shadow pinned to the box and
+     * attached `scroll`. So the pair of attachments *is* the feature,
+     * and it is what this asserts. The rendered result was measured in
+     * the harness (dark ramp 52,58,64 flat before; 52,57,63 at the last
+     * label and 22,24,27 at the bottom edge with more below; flat again
+     * at the end of the list) and is on the PR.
+     */
+    it('paints the fade only while there is more below', async () => {
+      const el = await surfaceWithPanel();
+
+      const wrapper = el.shadowRoot?.querySelector('wa-dialog');
+
+      await (wrapper as HTMLElement & { updateComplete: Promise<unknown> })
+        .updateComplete;
+
+      const body = wrapper?.shadowRoot?.querySelector('[part~="body"]');
+
+      expect(body, 'no body part to scroll').not.toBeNull();
+
+      const style = getComputedStyle(body as Element);
+
+      expect(style.overflowY, 'the body is what gives, not the cap').toBe(
+        'auto',
+      );
+
+      // The cover scrolls with the content; the shadow does not. Either
+      // one alone is a fade that is always there or never there.
+      expect(
+        style.backgroundAttachment,
+        'the cover must be local and the shadow must not',
+      ).toBe('local, scroll');
+
+      // Both sit at the bottom, or the cover hides nothing.
+      expect(style.backgroundPosition).toBe('50% 100%, 50% 100%');
+      expect(style.backgroundSize).toBe('100% 32px, 100% 32px');
+    });
+
     /**
      * A dialog with no accessible name is what `utils/name-dialog.ts`
      * exists for; here the name is already written on the panel, so no