fix(touch): enable annotation drawing and hover on touch devices

- Add touch-action:none to container when in annotate mode so the
  browser doesn't intercept touch gestures for scroll/pan
- Replace onmouseenter/onmouseleave with onpointerenter/onpointerleave
  so the highlight effect also fires on touch/stylus

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/AnnotationLayer.svelte

@@ -93,7 +93,7 @@ function handlePointerUp(event: PointerEvent) {
 let hoveredId = $state<string | null>(null);
 
 const containerStyle = $derived(
-	`position: absolute; top: 0; left: 0; width: 100%; height: 100%;${canAnnotate ? ' cursor: crosshair;' : ''}`
+	`position: absolute; top: 0; left: 0; width: 100%; height: 100%;${canAnnotate ? ' cursor: crosshair; touch-action: none;' : ''}`
 );
 </script>
 
@@ -113,8 +113,8 @@ const containerStyle = $derived(
 			aria-label="Kommentare anzeigen"
 			onclick={() => onAnnotationClick?.(annotation.id)}
 			onkeydown={(e) => { if (e.key === 'Enter' || e.key === ' ') onAnnotationClick?.(annotation.id); }}
-			onmouseenter={() => (hoveredId = annotation.id)}
-			onmouseleave={() => (hoveredId = null)}
+			onpointerenter={() => (hoveredId = annotation.id)}
+			onpointerleave={() => (hoveredId = null)}
 			style="
 				position: absolute;
 				left: {annotation.x * 100}%;