android: the floating back and more controls are bare glyphs with a shadow

Kyle's pick over the dark discs, seen on the artist page's hero: the
picture stays whole under them, and a two-layer drop shadow is what keeps
the chevron and the dots readable on a bright one. The tap target keeps its
size; a press shows a faint disc for feedback and nothing else.

src/mobile/shell.css

@@ -120,10 +120,13 @@
     display: grid;
     place-items: center;
     color: #fff;
-    background: rgba(0, 0, 0, 0.46);
-    box-shadow: inset 0 0 0 1px rgba(255, 255, 255, 0.08), 0 2px 10px rgba(0, 0, 0, 0.35);
+    /* bare glyphs with a shadow, Kyle's pick over the dark discs
+       (2026-09-18, the artist page's hero): the picture stays whole under
+       them, and a shadow is what keeps them readable on a bright one */
+    background: transparent;
+    filter: drop-shadow(0 1px 2px rgba(0, 0, 0, 0.9)) drop-shadow(0 2px 8px rgba(0, 0, 0, 0.55));
 }
-.fchrome:active { background: rgba(0, 0, 0, 0.62); }
+.fchrome:active { background: rgba(255, 255, 255, 0.14); }
 .fchrome.back { left: calc(10px + var(--inset-left)); }
 .fchrome.more { right: calc(10px + var(--inset-right)); color: rgba(255, 255, 255, 0.82); }