fix: iOS Safari AddToMenu visibility issue (#573)

- bump z-index above Player (backdrop: 199, dropdown: 200)
- add translateZ(0) to force GPU layer and fix iOS stacking context
- add explicit width constraints

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <[e-mail]>

frontend/src/lib/components/AddToMenu.svelte

···
 			left: 0;
 			right: 0;
 			bottom: 0;
-			z-index: 100;
+			z-index: 199;
 			background: rgba(0, 0, 0, 0.4);
+			-webkit-transform: translateZ(0);
+			transform: translateZ(0);
 		}
 
 		.menu-dropdown {
···
 			bottom: auto;
 			left: 0;
 			right: 0;
+			width: 100%;
 			min-width: 100%;
+			max-width: 100vw;
 			border-radius: 0 0 16px 16px;
 			padding-top: env(safe-area-inset-top, 0);
-			animation: slideDown 0.2s ease-out;
-			z-index: 101;
-		}
-
-		@keyframes slideDown {
-			from {
-				transform: translateY(-100%);
-			}
-			to {
-				transform: translateY(0);
-			}
+			z-index: 200;
+			-webkit-transform: translateZ(0);
+			transform: translateZ(0);
 		}
 
 		.menu-item {