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

Since #71 the phone's "More" is a bottom sheet, and at the reference
viewport it does not fit: measured at 424x439 with the seed's eight
destinations, the body is scrollHeight 412 against clientHeight 373, so
39px is below a fold nothing announces. Where the cut lands on a row
boundary the sheet ends in a clean edge that reads as the end of the
list, which is what #207 fixed one sheet over.

The rule is that sheet's, not a second answer to the same question:
#207's two background layers move into styles/sheet-scroll.css.ts and
both sheets adopt them, with the colour left to each host as
--yj-sheet-surface. The nav sheet paints the sidebar's --yj-bg-surface
and the context sheet the menus' --yj-bg-elevated, so a shared rule that
hard-coded either would draw that seam across the other one.

The half that makes it visible is that nothing inside the sheet may
repaint the surface. These are layers on the scroller, and app-sidebar's
host carries the same grey -- in the shell its own background, in the
sheet a second opaque copy of the sheet's, over the fade. With the
fragment adopted and that rule missing, the running app measured a flat
52,58,64 to the bottom edge with 39px still below: the defect unchanged,
with every assertion about background-attachment passing. menu-surface
already meets it from the other side, where the sheet's panel is
background-color: transparent.

Closes #210

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

@@ -254,6 +254,15 @@ describe('menu-surface', () => {
       // 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');
+
+      // The layers are shared with `bottom-nav`'s sheet since #210, and
+      // the colour is what each host still says for itself: this one
+      // paints the menus' `--yj-bg-elevated` (#343a40). A shared rule
+      // that hard-coded one grey would draw a seam across the other
+      // sheet, which is why the fragment reads a custom property.
+      expect(style.backgroundImage).toMatch(
+        /^linear-gradient\(rgb\(52, 58, 64\), rgb\(52, 58, 64\)\)/,
+      );
     });
 
     /**