fix: proper z-index stacking and viewport-aware positioning for likers tooltip (#484)

z-index fix:
- remove static z-index from .likes element
- only apply z-index: 10 when tooltip is open (.likes.tooltip-open)
- this ensures the active tooltip's parent elevates above sibling tracks
without all tracks competing at the same z-index level

flip detection:
- detect when likes element is <300px from viewport top
- flip tooltip to appear below instead of above
- prevents tooltip from colliding with sticky header

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

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

frontend/src/lib/components/LikersTooltip.svelte

···
 	let likers = $state<Liker[]>([]);
 	let loading = $state(true); // start as loading
 	let error = $state<string | null>(null);
+	let tooltipElement: HTMLDivElement | null = $state(null);
+	let positionBelow = $state(false);
+
+	// check if tooltip should flip below based on viewport position
+	$effect(() => {
+		if (!tooltipElement) return;
+
+		// get the parent element's position (the .likes span)
+		const parent = tooltipElement.parentElement;
+		if (!parent) return;
+
+		const parentRect = parent.getBoundingClientRect();
+		// if less than 300px from viewport top, flip tooltip below
+		// 300px accounts for tooltip max height (~240px) plus padding
+		positionBelow = parentRect.top < 300;
+	});
 
 	$effect(() => {
 		if (likeCount === 0) {
···
 </script>
 
 <div
+	bind:this={tooltipElement}
 	class="likers-tooltip"
+	class:position-below={positionBelow}
 	role="tooltip"
 	onmouseenter={onMouseEnter}
 	onmouseleave={onMouseLeave}
···
 		box-shadow: 0 4px 12px rgba(0, 0, 0, 0.5);
 		z-index: 1000;
 		pointer-events: auto;
+	}
+
+	/* flip tooltip below when near viewport top */
+	.likers-tooltip.position-below {
+		bottom: auto;
+		top: 100%;
+		margin-bottom: 0;
+		margin-top: 0.5rem;
 	}
 
 	.loading,

frontend/src/lib/components/TrackItem.svelte

···
 				<span class="meta-separator">•</span>
 				<span
 					class="likes"
+					class:tooltip-open={showLikersTooltip}
 					role="button"
 					tabindex="0"
 					aria-label={`${likeCount} ${likeCount === 1 ? 'like' : 'likes'} (focus to view users)`}
···
 		position: relative;
 		cursor: help;
 		transition: color 0.2s;
-		z-index: 100;
+	}
+
+	/* only elevate z-index when tooltip is open - this ensures the open tooltip
+	   renders above sibling track items without all tracks competing for z-index */
+	.likes.tooltip-open {
+		z-index: 10;
 	}
 
 	.likes:hover {